diff --git a/src/core/graph/NodeContext.ts b/src/core/graph/NodeContext.ts
index 3e436ea1..516043a0 100644
--- a/src/core/graph/NodeContext.ts
+++ b/src/core/graph/NodeContext.ts
@@ -240,6 +240,7 @@ export interface NodeContext {
    * the one case where the rule is in the way: the death sound of an object that destroys itself.
    * `fx` names a sound FX asset the voice is played through (#311); `''` plays it as it is.
    * `fxParams` are values for its parameters; one not given is the default the FX declares.
+   * Returns the number of the voice for `tuneSound`, or 0 where nothing is played.
    */
   playSound(
     assetId: string,
@@ -248,9 +249,14 @@ export interface NodeContext {
     detached: boolean,
     fx: string,
     fxParams: FxParams,
-  ): void;
+  ): number;
   /** Silences every voice of that sound; `''` silences everything this run started. */
   stopSound(assetId: string): void;
+  /**
+   * Hands new values to the sound FX parameters of one voice, by the number `playSound` gave it
+   * (#315), so a running sound follows the game. A voice that has ended is not reached.
+   */
+  tuneSound(voice: number, fxParams: FxParams): void;
   /**
    * Asks for another scene, by asset id. The swap happens once the frame is over – a graph must be
    * able to finish the flow it is in, and half a frame in the old world and half in the new one is
diff --git a/src/core/graph/nodes/action/PlaySound.ts b/src/core/graph/nodes/action/PlaySound.ts
index 407df093..34bc626e 100644
--- a/src/core/graph/nodes/action/PlaySound.ts
+++ b/src/core/graph/nodes/action/PlaySound.ts
@@ -59,7 +59,9 @@ export const PlaySound: NodeDef = {
     'the node only colors it. Each play is a voice of its own: firing this twice in a row is heard ' +
     'twice, overlapping. The sound belongs to this object and stops when the object is destroyed, ' +
     'unless "detached" is on. "fx" sends it through a sound FX asset; left empty, it plays as it is. ' +
-    '"fx params" takes values for its parameters from a Sound FX parameters node.',
+    '"fx params" takes values for its parameters from a Sound FX parameters node. "voice" is the ' +
+    'number of this one play, for Set sound FX parameters to change it while it plays; 0 where ' +
+    'nothing was played.',
   inputs: [
     exec(),
     soundPort(),
@@ -69,25 +71,26 @@ export const PlaySound: NodeDef = {
     soundFxPort(),
     { key: 'fxParams', label: 'fx params', type: 'fxparams' },
   ],
-  outputs: [exec()],
+  outputs: [exec(), num('voice')],
   params: [],
   run: (ctx, inputs) => {
     const sound = asString(inputs.sound);
-    if (sound)
-      ctx.playSound(
-        sound,
-        asNumber(inputs.volume),
-        loopChoice(inputs.loop),
-        asBoolean(inputs.detached),
-        asString(inputs.fx),
-        fxParamsOf(inputs.fxParams),
-      );
-    return { next: 'exec' };
+    const voice = sound
+      ? ctx.playSound(
+          sound,
+          asNumber(inputs.volume),
+          loopChoice(inputs.loop),
+          asBoolean(inputs.detached),
+          asString(inputs.fx),
+          fxParamsOf(inputs.fxParams),
+        )
+      : 0;
+    return { next: 'exec', outputs: { voice } };
   },
 };
 
 /** The numbers of a parameter bundle; anything else hands in none. */
-function fxParamsOf(value: Value | undefined): FxParams {
+export function fxParamsOf(value: Value | undefined): FxParams {
   if (!isStruct(value)) return {};
   const params: Record<string, number> = {};
   for (const [key, item] of Object.entries(value)) if (typeof item === 'number') params[key] = item;
diff --git a/src/core/graph/nodes/action/SetSoundFxParameters.ts b/src/core/graph/nodes/action/SetSoundFxParameters.ts
new file mode 100644
index 00000000..5c9c3fdc
--- /dev/null
+++ b/src/core/graph/nodes/action/SetSoundFxParameters.ts
@@ -0,0 +1,32 @@
+import type { NodeDef } from '../../NodeRegistry';
+import { asNumber } from '../../values';
+import { exec, num } from '../shared';
+import { fxParamsOf } from './PlaySound';
+
+/**
+ * Changes the sound FX parameters of a sound that is already playing (#315). It reaches one voice,
+ * by the number its `Play sound` handed out: every play is a sound of its own, and two objects with
+ * the same engine sound – or one object playing it twice – each keep theirs.
+ */
+export const SetSoundFxParameters: NodeDef = {
+  type: 'SetSoundFxParameters',
+  label: 'Set sound FX parameters',
+  category: 'action',
+  description:
+    'Changes the sound FX parameters of a sound while it plays: the voice a Play sound handed out ' +
+    'glides to the values from a Sound FX parameters node. Keep the voice in a variable to reach it ' +
+    'from another event. A parameter not given keeps its value, a voice that has ended is not ' +
+    'reached. A pitch changes by speed here, so the sound gets shorter or longer while it is moved.',
+  inputs: [
+    exec(),
+    { ...num('voice'), inline: false },
+    { key: 'fxParams', label: 'fx params', type: 'fxparams' },
+  ],
+  outputs: [exec()],
+  params: [],
+  run: (ctx, inputs) => {
+    const voice = asNumber(inputs.voice);
+    if (voice > 0) ctx.tuneSound(voice, fxParamsOf(inputs.fxParams));
+    return { next: 'exec' };
+  },
+};
diff --git a/src/core/graph/nodes/data/SoundFxParameters.ts b/src/core/graph/nodes/data/SoundFxParameters.ts
index e7c598d9..61b2a1cc 100644
--- a/src/core/graph/nodes/data/SoundFxParameters.ts
+++ b/src/core/graph/nodes/data/SoundFxParameters.ts
@@ -6,14 +6,14 @@ import { asNumber } from '../../values';
 /**
  * Values for the parameters of one sound FX (#311), picked from those its Input declares the way a
  * Variables node picks variables. Each picked one is a number input starting at the FX's default;
- * the bundle goes into the `fx params` input of a `Play sound`.
+ * the bundle goes into the `fx params` input of a `Play sound` or a `Set sound FX parameters`.
  */
 export const SoundFxParameters: NodeDef = {
   type: SOUND_FX_PARAMETERS,
   label: 'Sound FX parameters',
   category: 'data',
   description:
-    'Values for the parameters of a sound FX, for the "fx params" input of a Play sound. Pick the parameters to set; one not picked keeps the default its Input gives it.',
+    'Values for the parameters of a sound FX, for the "fx params" input of a Play sound or a Set sound FX parameters. Pick the parameters to set; one not picked keeps the default its Input gives it, or on a playing sound the value it has.',
   inputs: [],
   outputs: [{ key: 'params', label: 'params', type: 'fxparams' }],
   params: [SOUND_FX_SETTING],
diff --git a/src/core/graph/nodes/index.ts b/src/core/graph/nodes/index.ts
index 13758769..dcbe6ae5 100644
--- a/src/core/graph/nodes/index.ts
+++ b/src/core/graph/nodes/index.ts
@@ -23,6 +23,7 @@ import { Raycast } from './action/Raycast';
 import { RaycastAll } from './action/RaycastAll';
 import { RestartGame } from './action/RestartGame';
 import { SetPosition } from './action/SetPosition';
+import { SetSoundFxParameters } from './action/SetSoundFxParameters';
 import { SetZ } from './action/SetZ';
 import { Spawn } from './action/Spawn';
 import { StopGame } from './action/StopGame';
@@ -298,6 +299,7 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   StopAnimation,
   PlaySound,
   StopSound,
+  SetSoundFxParameters,
   SoundFxParameters,
   LoadScene,
   Log,
diff --git a/src/core/soundfx/nodes.ts b/src/core/soundfx/nodes.ts
index 818128dc..12e07678 100644
--- a/src/core/soundfx/nodes.ts
+++ b/src/core/soundfx/nodes.ts
@@ -50,6 +50,14 @@ export const EMPTY_PLAN: FxPlan = { pitch: [], stages: [] };
 /** Values for the parameters, by name; a name the plan does not have is left out. */
 export type FxParams = Readonly<Record<string, number>>;
 
+/** How a voice goes through its sound FX: the plan and the values it starts with, kept so they can change while it plays (#315). */
+export interface FxVoice {
+  plan: FxPlan;
+  params: FxParams;
+}
+
+export const DRY_VOICE: FxVoice = { plan: EMPTY_PLAN, params: {} };
+
 /**
  * A node of a sound FX graph (#311). It never runs: `emit` says what it does to the sounds arriving
  * at its audio inputs, by input key, given the numbers at its other inputs. Input and Output have no
@@ -239,23 +247,24 @@ function amountOf(amount: FxAmount, params: FxParams): number {
   return Math.max(amount.min ?? -Infinity, Math.min(amount.max ?? Infinity, given));
 }
 
+/** A stage fed by a parameter stays at 0 as well: a value handed in later has to find it there. */
 function stagesOf(plan: FxPlan, params: FxParams): FxStage[] {
   const stages: FxStage[] = [];
   for (const stage of plan.stages) {
     if (stage.kind === 'gain') {
       const db = amountOf(stage.db, params);
-      if (db !== 0) stages.push({ kind: 'gain', gain: 10 ** (db / 20) });
+      if (db !== 0 || stage.db.param !== undefined) stages.push({ kind: 'gain', gain: 10 ** (db / 20) });
       continue;
     }
     const gain = amountOf(stage.gain, params);
-    if (gain !== 0) stages.push({ ...stage, gain });
+    if (gain !== 0 || stage.gain.param !== undefined) stages.push({ ...stage, gain });
   }
   return stages;
 }
 
 /**
- * The chain one voice is sent along: the plan with every parameter filled in. A Mix that ends at 0
- * or 1 is left out with the sound it silences, so that one is not even started.
+ * The chain one voice is sent along: the plan with every parameter filled in. Its shape depends on
+ * the plan alone, never on the values, so the values of a playing voice can change in place (#315).
  */
 export function fxChainOf(plan: FxPlan, params: FxParams = {}): FxChain {
   const stages = stagesOf(plan, params);
@@ -263,10 +272,10 @@ export function fxChainOf(plan: FxPlan, params: FxParams = {}): FxChain {
     const pitch = plan.pitch.reduce((sum, amount) => sum + amountOf(amount, params), 0);
     return { pitch, stages };
   }
-  const mix = amountOf(plan.mix, params);
-  if (mix <= 0 || mix >= 1) {
-    const heard = fxChainOf(mix <= 0 ? plan.a : plan.b, params);
-    return { ...heard, stages: [...heard.stages, ...stages] };
-  }
-  return { mix, a: fxChainOf(plan.a, params), b: fxChainOf(plan.b, params), stages };
+  return {
+    mix: amountOf(plan.mix, params),
+    a: fxChainOf(plan.a, params),
+    b: fxChainOf(plan.b, params),
+    stages,
+  };
 }
diff --git a/src/editor/debugHook.ts b/src/editor/debugHook.ts
index bd8842be..0e6d5d8e 100644
--- a/src/editor/debugHook.ts
+++ b/src/editor/debugHook.ts
@@ -41,6 +41,8 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
     sound: {
       stats: () => soundLibrary.stats(),
       duration: (file: string) => soundLibrary.duration(file),
+      /** Where the knobs of each voice of that file are now, oldest first, a pitch as its speed (#315). */
+      levels: (file: string) => soundLibrary.levels(file),
       /** Decodes ahead of a check, so a play can be asserted without racing the network. */
       load: (file: string) => soundLibrary.load(file).then((buffer) => buffer?.duration ?? null),
       stopAll: () => soundLibrary.stopAll(),
diff --git a/src/editor/soundfx/SoundFxEditor.ts b/src/editor/soundfx/SoundFxEditor.ts
index 20259b93..65c8023e 100644
--- a/src/editor/soundfx/SoundFxEditor.ts
+++ b/src/editor/soundfx/SoundFxEditor.ts
@@ -3,7 +3,6 @@ import { findAssetOf } from '../../core/model/Project';
 import { resolveAssetProps } from '../../core/properties/resolve';
 import { volumeOf } from '../../core/properties/sounds';
 import { soundFxBuildFor, soundFxParams } from '../../core/soundfx/compile';
-import { fxChainOf } from '../../core/soundfx/nodes';
 import { Button, Dropdown, HStack, Label, Menu, NumberField, Stack, Theme } from '../../ui';
 import { soundLibrary } from '../../viewport/SoundLibrary';
 import { AssetTabEditor } from '../AssetTabEditor';
@@ -40,6 +39,8 @@ export class SoundFxEditor extends AssetTabEditor<SoundFxAsset, GraphCanvas> {
   private defaults: Record<string, number> = {};
   private paramsKey = '';
   private valuesFor = '';
+  /** The voices the tab started that may still be playing – what a moved field reaches. */
+  private voices: number[] = [];
 
   constructor(state: EditorState) {
     super(state, { infoWidth: INFO_WIDTH });
@@ -89,8 +90,8 @@ export class SoundFxEditor extends AssetTabEditor<SoundFxAsset, GraphCanvas> {
   /** Sets one parameter the header plays with, as typing into its field does. */
   setPreviewValue(key: string, value: number): void {
     if (!(key in this.values)) return;
-    this.values[key] = value;
     this.paramFields.get(key)?.setNumber(value);
+    this.setValue(key, value);
   }
 
   /** Plays the picked sound through the graph; a graph that does not compile is not played at all. */
@@ -100,11 +101,22 @@ export class SoundFxEditor extends AssetTabEditor<SoundFxAsset, GraphCanvas> {
     const plan = asset && soundFxBuildFor(asset).compiled;
     if (!sound?.file || !plan) return;
     const volume = volumeOf(resolveAssetProps(this.state.project, sound));
-    soundLibrary.play(sound.file, volume, false, PREVIEW_OWNER, fxChainOf(plan, this.values));
+    const voice = soundLibrary.play(sound.file, volume, false, PREVIEW_OWNER, {
+      plan,
+      params: { ...this.values },
+    });
+    if (voice) this.voices.push(voice);
+  }
+
+  /** A moved field is heard at once on what the tab is playing (#315); an ended voice is let go. */
+  private setValue(key: string, value: number): void {
+    this.values[key] = value;
+    this.voices = this.voices.filter((voice) => soundLibrary.tune(voice, { [key]: value }));
   }
 
   stop(): void {
     soundLibrary.stopOwner(PREVIEW_OWNER);
+    this.voices = [];
   }
 
   protected override infoText(asset: SoundFxAsset): string {
@@ -152,8 +164,8 @@ export class SoundFxEditor extends AssetTabEditor<SoundFxAsset, GraphCanvas> {
         const field = new NumberField(values[port.key], {
           width: PARAM_WIDTH,
           step: 0.1,
-          onInput: (value) => (this.values[port.key] = value),
-          onChange: (value) => (this.values[port.key] = value),
+          onInput: (value) => this.setValue(port.key, value),
+          onChange: (value) => this.setValue(port.key, value),
         });
         this.paramFields.set(port.key, field);
         this.paramBar.add(field);
diff --git a/src/runtime/systems/Audio.ts b/src/runtime/systems/Audio.ts
index b2977a0a..b2b6a77e 100644
--- a/src/runtime/systems/Audio.ts
+++ b/src/runtime/systems/Audio.ts
@@ -1,4 +1,4 @@
-import type { FxChain } from '../../core/soundfx/nodes';
+import type { FxParams, FxVoice } from '../../core/soundfx/nodes';
 
 /**
  * What the runtime needs of the outside world to make a noise – the counterpart of `Input` on the
@@ -13,10 +13,17 @@ export interface AudioSink {
    * Starts one voice. `volume` is final (0–1) and every call overlaps whatever is already playing.
    * `owner` is the behavior the sound belongs to: when that behavior is destroyed the voice goes
    * with it, because a sound is something a thing in the world is doing, not a thing of its own.
-   * `''` means the voice has no owner and outlives whatever started it. `fx` is the line of effects
-   * the voice passes on its way out (#311); `DRY` passes none.
+   * `''` means the voice has no owner and outlives whatever started it. `fx` is the sound FX the
+   * voice passes on its way out (#311), with the values of its parameters; `DRY_VOICE` passes none.
+   * Returns the number of the voice, never used twice, or 0 where nothing is going to play.
    */
-  play(file: string, volume: number, loop: boolean, owner: string, fx?: FxChain): void;
+  play(file: string, volume: number, loop: boolean, owner: string, fx?: FxVoice): number;
+  /**
+   * Hands new values to the sound FX parameters of one voice, by the number `play` gave it (#315).
+   * A name its FX does not have is left out, one not given keeps its value. A voice that has ended
+   * is not there any more and nothing happens – a detached one is reached like any other.
+   */
+  tune(voice: number, params: FxParams): void;
   /** Silences every voice of that file; `''` silences all of them. */
   stop(file: string): void;
   /** Silences every voice started by that owner – what destroying a behavior does. */
@@ -24,7 +31,8 @@ export interface AudioSink {
 }
 
 export const SILENT_AUDIO: AudioSink = {
-  play: () => {},
+  play: () => 0,
+  tune: () => {},
   stop: () => {},
   stopOwner: () => {},
 };
diff --git a/src/runtime/systems/behaviorContext.ts b/src/runtime/systems/behaviorContext.ts
index 0cfa19ab..bce400fe 100644
--- a/src/runtime/systems/behaviorContext.ts
+++ b/src/runtime/systems/behaviorContext.ts
@@ -16,7 +16,7 @@ import { hasRenderLayer } from '../../core/properties/PropertySchema';
 import { resolveAssetProps } from '../../core/properties/resolve';
 import { loopsOf, volumeOf } from '../../core/properties/sounds';
 import { soundFxBuildFor } from '../../core/soundfx/compile';
-import { DRY, fxChainOf, type FxParams } from '../../core/soundfx/nodes';
+import { DRY_VOICE, type FxParams } from '../../core/soundfx/nodes';
 import type { Entity } from '../Entity';
 import { PhysicsSystem } from '../physics/PhysicsSystem';
 import type { World } from '../World';
@@ -172,6 +172,7 @@ export function behaviorContext(
       const file = assetId === '' ? '' : soundFileOf(project, assetId);
       if (assetId === '' || file) run.audio.stop(file);
     },
+    tuneSound: (voice, fxParams) => run.audio.tune(voice, fxParams),
     spawn: (assetId, position, parent) => world.spawn(assetId, position, resolve(parent) ?? null),
     addJoint: (assetId, owner, connected, at) => {
       const on = resolve(owner);
@@ -218,7 +219,7 @@ export function behaviorContext(
  * node's, and its loop flag unless the node overrules it either way. The asset is what a sound
  * *is* – a node that says nothing therefore plays it as the project describes it. An FX asset that
  * does not compile plays the sound dry: a broken effect should not silence the game. Its parameters
- * take the values handed in, the others their defaults.
+ * take the values handed in, the others their defaults. Returns the number of the voice, 0 for none.
  */
 function playSound(
   { project, audio }: ContextSources,
@@ -228,14 +229,20 @@ function playSound(
   owner: string,
   fxId: string,
   fxParams: FxParams,
-): void {
+): number {
   const asset = findAssetOf(project, assetId, 'sound');
-  if (!asset?.file) return;
+  if (!asset?.file) return 0;
   const props = resolveAssetProps(project, asset);
   const level = Math.max(0, Math.min(1, volumeOf(props) * volume));
   const fx = fxId ? findAssetOf(project, fxId, 'soundfx') : undefined;
   const plan = fx && soundFxBuildFor(fx).compiled;
-  audio.play(asset.file, level, loop ?? loopsOf(props), owner, plan ? fxChainOf(plan, fxParams) : DRY);
+  return audio.play(
+    asset.file,
+    level,
+    loop ?? loopsOf(props),
+    owner,
+    plan ? { plan, params: fxParams } : DRY_VOICE,
+  );
 }
 
 /** Where a ray of that length ends; where it has no direction, where it starts. */
diff --git a/src/viewport/SoundLibrary.ts b/src/viewport/SoundLibrary.ts
index 63b85aef..0cbe41b3 100644
--- a/src/viewport/SoundLibrary.ts
+++ b/src/viewport/SoundLibrary.ts
@@ -1,34 +1,62 @@
-import { DRY, type FxChain, type FxStage } from '../core/soundfx/nodes';
+import {
+  DRY_VOICE,
+  fxChainOf,
+  type FxChain,
+  type FxParams,
+  type FxStage,
+  type FxVoice,
+} from '../core/soundfx/nodes';
 import { pitchShift } from '../core/soundfx/pitchShift';
 import type { AudioSink } from '../runtime/systems/Audio';
 
 const DEFAULT_BASE = '/game-maker/data/sounds';
 /** How many pitched copies are kept at once. */
 const SHIFTED_KEPT = 32;
+/** Seconds a changed parameter takes to get most of the way there: quick, but no click (#315). */
+const GLIDE = 0.03;
+
+/**
+ * One number of a voice that can still change while it plays (#315): a level, a share of a Mix, the
+ * gain of a band – or the pitch of a source, which glides by speed from the pitch its copy was made at.
+ */
+type Knob =
+  | { kind: 'level'; param: AudioParam; target: number }
+  | { kind: 'pitch'; source: AudioBufferSourceNode; base: number; target: number };
 
 /**
  * One sound in flight: its sources, its own gain, the file it came from and who is making it. A
- * voice has a source per pitch its sound FX plays the file at (#316), started together and as long
- * as each other.
+ * voice has a source per way through its sound FX (#316), started together and as long as each
+ * other. Its knobs are in the order `levelsOf` lists the numbers of its chain.
  */
 interface Voice {
+  /** What `play` returned for it, the handle a `tune` reaches it by. */
+  id: number;
   file: string;
   /** The behavior this voice belongs to; `''` for one nothing owns (see `AudioSink.play`). */
   owner: string;
   sources: AudioBufferSourceNode[];
   gain: GainNode;
+  fx: FxVoice;
+  knobs: Knob[];
 }
 
-/** What one voice is wired from: the file, and its sources by pitch so far. */
+/** What one voice is wired from: the file, and what has been made for it so far. */
 interface Wiring {
   context: AudioContext;
   file: string;
   buffer: AudioBuffer;
   loop: boolean;
-  sources: Map<number, AudioBufferSourceNode>;
+  sources: { source: AudioBufferSourceNode; pitch: number }[];
+  knobs: Knob[];
   stages: number;
 }
 
+/** A play still waiting on its bytes: what a `tune` in the meantime writes into. */
+interface Pending {
+  id: number;
+  fx: FxVoice;
+}
+
 /**
  * Plays uploaded sounds, keyed by file name – the texture library's twin on the audio side. Files
  * are content addressed, so a decoded buffer can never go stale and nothing is evicted.
@@ -52,8 +80,12 @@ export class SoundLibrary implements AudioSink {
   /** Never retried, so one broken reference cannot spam the network. */
   private readonly failed = new Set<string>();
   private voices: Voice[] = [];
+  private pending: Pending[] = [];
+  /** The number the next play is given; 0 stays free for "nothing plays". */
+  private nextId = 1;
   private started = 0;
   private stopped = 0;
+  private tuned = 0;
   /** The last voice actually started, with the values it was started at – what a check reads back. */
   private last: LastVoice | null = null;
   /**
@@ -73,20 +105,44 @@ export class SoundLibrary implements AudioSink {
     this.base = base.replace(/\/+$/, '');
   }
 
-  play(file: string, volume: number, loop: boolean, owner = '', fx: FxChain = DRY): void {
-    if (file === '' || this.failed.has(file)) return;
+  /**
+   * The number is handed out at once, before the bytes are there: a graph can tune a voice in the
+   * same frame it asked for it, and the voice then starts with those values.
+   */
+  play(file: string, volume: number, loop: boolean, owner = '', fx: FxVoice = DRY_VOICE): number {
+    if (file === '' || this.failed.has(file)) return 0;
+    const id = this.nextId++;
     const buffer = this.buffers.get(file);
     if (buffer) {
-      this.start(file, buffer, volume, loop, owner, fx);
-      return;
+      this.start(id, file, buffer, volume, loop, owner, fx);
+      return id;
     }
     const askedAt = this.cancelsFor(file, owner);
+    const waiting: Pending = { id, fx: { plan: fx.plan, params: { ...fx.params } } };
+    this.pending.push(waiting);
     void this.load(file).then((loaded) => {
+      this.pending = this.pending.filter((other) => other !== waiting);
       // A `Stop sound` – or the destruction of the behavior that asked – that happened while the
       // bytes were in flight has to win, or a sound the run already ended would start afterwards.
       if (loaded && this.cancelsFor(file, owner) === askedAt)
-        this.start(file, loaded, volume, loop, owner, fx);
+        this.start(id, file, loaded, volume, loop, owner, waiting.fx);
     });
+    return id;
+  }
+
+  /** Says whether the voice was still there to be reached – playing, or waiting on its bytes. */
+  tune(id: number, params: FxParams): boolean {
+    const waiting = this.pending.find((other) => other.id === id);
+    if (waiting) {
+      waiting.fx = { plan: waiting.fx.plan, params: { ...waiting.fx.params, ...params } };
+      return true;
+    }
+    const voice = this.voices.find((other) => other.id === id);
+    if (!voice) return false;
+    voice.fx = { plan: voice.fx.plan, params: { ...voice.fx.params, ...params } };
+    this.glide(voice);
+    this.tuned += 1;
+    return true;
   }
 
   stop(file: string): void {
@@ -123,6 +179,7 @@ export class SoundLibrary implements AudioSink {
     playing: number;
     started: number;
     stopped: number;
+    tuned: number;
     decoded: number;
     failed: number;
     last: LastVoice | null;
@@ -131,12 +188,25 @@ export class SoundLibrary implements AudioSink {
       playing: this.voices.length,
       started: this.started,
       stopped: this.stopped,
+      tuned: this.tuned,
       decoded: this.buffers.size,
       failed: this.failed.size,
       last: this.last,
     };
   }
 
+  /**
+   * Where the knobs of each voice of that file are right now, oldest first, a pitch as the speed it
+   * plays at – what a check reads to see a change arrive at one voice and not another (test hook).
+   */
+  levels(file: string): number[][] {
+    return this.voices
+      .filter((voice) => voice.file === file)
+      .map((voice) =>
+        voice.knobs.map((knob) => (knob.kind === 'pitch' ? knob.source.playbackRate : knob.param).value),
+      );
+  }
+
   /** How long a decoded sound runs, in seconds; null while it has not been loaded (test hook). */
   duration(file: string): number | null {
     return this.buffers.get(file)?.duration ?? null;
@@ -177,12 +247,13 @@ export class SoundLibrary implements AudioSink {
   }
 
   private start(
+    id: number,
     file: string,
     buffer: AudioBuffer,
     volume: number,
     loop: boolean,
     owner: string,
-    fx: FxChain,
+    fx: FxVoice,
   ): void {
     const context = this.audio();
     // A context starts suspended where the page has not been interacted with; without this the
@@ -190,18 +261,18 @@ export class SoundLibrary implements AudioSink {
     if (context.state === 'suspended') void context.resume();
     const gain = context.createGain();
     gain.gain.value = Math.max(0, Math.min(1, volume));
-    const wiring: Wiring = { context, file, buffer, loop, sources: new Map(), stages: 0 };
-    this.wire(wiring, fx).connect(gain).connect(context.destination);
-    const sources = [...wiring.sources.values()];
+    const wiring: Wiring = { context, file, buffer, loop, sources: [], knobs: [], stages: 0 };
+    this.wire(wiring, fxChainOf(fx.plan, fx.params)).connect(gain).connect(context.destination);
+    const sources = wiring.sources.map((made) => made.source);
     this.last = {
       file,
       volume: gain.gain.value,
       loop,
-      pitches: [...wiring.sources.keys()],
+      pitches: wiring.sources.map((made) => made.pitch),
       stages: wiring.stages,
       duration: sources[0]!.buffer!.duration,
     };
-    const voice: Voice = { file, owner, sources, gain };
+    const voice: Voice = { id, file, owner, sources, gain, fx, knobs: wiring.knobs };
     let playing = sources.length;
     for (const source of sources)
       source.onended = () => {
@@ -213,35 +284,60 @@ export class SoundLibrary implements AudioSink {
   }
 
   /**
-   * The nodes a chain becomes, up to the one that carries its sound on. Two ways at the same pitch
-   * share a source; a Mix weighs each of its sounds by its share before they are summed.
+   * The nodes a chain becomes, up to the one that carries its sound on. Every way has a source of
+   * its own, so each can glide to its own pitch; a Mix weighs each of its sounds by its share before
+   * they are summed.
    */
   private wire(wiring: Wiring, chain: FxChain): AudioNode {
-    const { context } = wiring;
+    const { context, knobs } = wiring;
     let tail: AudioNode;
     if ('mix' in chain) {
       tail = context.createGain();
-      this.wire(wiring, chain.a)
-        .connect(gainNode(context, 1 - chain.mix))
-        .connect(tail);
-      this.wire(wiring, chain.b).connect(gainNode(context, chain.mix)).connect(tail);
+      const wayA = this.wire(wiring, chain.a);
+      const shareA = gainNode(context, 1 - chain.mix);
+      wayA.connect(shareA).connect(tail);
+      knobs.push({ kind: 'level', param: shareA.gain, target: 1 - chain.mix });
+      const wayB = this.wire(wiring, chain.b);
+      const shareB = gainNode(context, chain.mix);
+      wayB.connect(shareB).connect(tail);
+      knobs.push({ kind: 'level', param: shareB.gain, target: chain.mix });
     } else tail = this.source(wiring, chain.pitch);
-    for (const stage of chain.stages) tail = tail.connect(fxNode(context, stage));
+    for (const stage of chain.stages) {
+      const [node, param] = fxNode(context, stage);
+      tail = tail.connect(node);
+      knobs.push({ kind: 'level', param, target: stage.gain });
+    }
     wiring.stages += chain.stages.length;
     return tail;
   }
 
   private source(wiring: Wiring, pitch: number): AudioBufferSourceNode {
-    const held = wiring.sources.get(pitch);
-    if (held) return held;
     const { context, file, buffer, loop } = wiring;
     const source = context.createBufferSource();
     source.buffer = pitch === 0 ? buffer : this.pitched(context, file, buffer, pitch, loop);
     source.loop = loop;
-    wiring.sources.set(pitch, source);
+    wiring.sources.push({ source, pitch });
+    wiring.knobs.push({ kind: 'pitch', source, base: pitch, target: pitch });
     return source;
   }
 
+  /**
+   * Sends every knob of a voice towards what its values say now. A pitch cannot get a new copy in
+   * the middle of a sound, so it glides by speed: the voice gets that much shorter or longer.
+   */
+  private glide(voice: Voice): void {
+    const levels = levelsOf(fxChainOf(voice.fx.plan, voice.fx.params));
+    const now = this.audio().currentTime;
+    voice.knobs.forEach((knob, i) => {
+      const level = levels[i];
+      if (level === undefined || level === knob.target) return;
+      knob.target = level;
+      if (knob.kind === 'pitch')
+        knob.source.playbackRate.setTargetAtTime(2 ** ((level - knob.base) / 12), now, GLIDE);
+      else knob.param.setTargetAtTime(level, now, GLIDE);
+    });
+  }
+
   /**
    * The sound at another pitch and its own length (#312), worked out once per file, pitch and loop
    * and kept for the next voice. The oldest goes first, so a pitch drawn at random every play cannot
@@ -286,7 +382,7 @@ interface LastVoice {
   file: string;
   volume: number;
   loop: boolean;
-  /** Semitones of each source, one per pitch (#316), and how many gain and filter nodes it passed. */
+  /** Semitones of each source, one per way (#316), and how many gain and filter nodes it passed. */
   pitches: number[];
   stages: number;
   /** Seconds of what was played: a pitched voice is as long as the sound itself (#312). */
@@ -299,14 +395,30 @@ function gainNode(context: AudioContext, level: number): GainNode {
   return gain;
 }
 
-function fxNode(context: AudioContext, stage: FxStage): AudioNode {
-  if (stage.kind === 'gain') return gainNode(context, stage.gain);
+/** The node a stage becomes, with the number of it a parameter can move. */
+function fxNode(context: AudioContext, stage: FxStage): [AudioNode, AudioParam] {
+  if (stage.kind === 'gain') {
+    const gain = gainNode(context, stage.gain);
+    return [gain, gain.gain];
+  }
   const filter = context.createBiquadFilter();
   filter.type = stage.filter;
   filter.frequency.value = stage.frequency;
   filter.gain.value = stage.gain;
   filter.Q.value = stage.q;
-  return filter;
+  return [filter, filter.gain];
+}
+
+/** The numbers of a chain in the order `wire` makes their knobs: a stage's `gain` is what its node holds. */
+function levelsOf(chain: FxChain, into: number[] = []): number[] {
+  if ('mix' in chain) {
+    levelsOf(chain.a, into);
+    into.push(1 - chain.mix);
+    levelsOf(chain.b, into);
+    into.push(chain.mix);
+  } else into.push(chain.pitch);
+  for (const stage of chain.stages) into.push(stage.gain);
+  return into;
 }
 
 export const soundLibrary = new SoundLibrary();
diff --git a/tests/core/keep-on-scene-load.test.ts b/tests/core/keep-on-scene-load.test.ts
index b7abd05e..818e9466 100644
--- a/tests/core/keep-on-scene-load.test.ts
+++ b/tests/core/keep-on-scene-load.test.ts
@@ -54,7 +54,8 @@ function keep(f: Fixture, sceneId: string, objectId: string): void {
 function start(f: Fixture, audio?: AudioSink) {
   const stoppedOwners: string[] = [];
   const sink: AudioSink = audio ?? {
-    play: () => {},
+    play: () => 0,
+    tune: () => {},
     stop: () => {},
     stopOwner: (owner: string) => stoppedOwners.push(owner),
   };
diff --git a/tests/core/node-catalog.test.ts b/tests/core/node-catalog.test.ts
index 9506d378..c1a1c7c9 100644
--- a/tests/core/node-catalog.test.ts
+++ b/tests/core/node-catalog.test.ts
@@ -1940,12 +1940,14 @@ const CASES: Record<string, Case[]> = {
       name: 'no sound is a no-op that still continues the flow',
       inputs: { sound: '', volume: 1, loop: '' },
       next: 'exec',
+      outputs: { voice: 0 },
       after: (ctx) => expect(ctx.played).toEqual([]),
     },
     {
-      name: 'hands the id on with the volume and loop it was given',
+      name: 'hands the id on with the volume and loop it was given, and gives out the voice (#315)',
       inputs: { sound: 'snd-1', volume: 0.5, loop: 'true' },
       next: 'exec',
+      outputs: { voice: 1 },
       // What the two mean – a factor on the asset's volume, an answer that overrules its loop
       // flag – is the BehaviorSystem's to resolve, since only it holds the project (see
       // `sounds.test.ts`).
@@ -2025,6 +2027,20 @@ const CASES: Record<string, Case[]> = {
       after: (ctx) => expect(ctx.silenced).toEqual(['']),
     },
   ],
+  SetSoundFxParameters: [
+    {
+      name: 'hands the numbers of the bundle to the voice it is given',
+      inputs: { voice: 4, fxParams: { depth: 3, label: 'x' } },
+      next: 'exec',
+      after: (ctx) => expect(ctx.tuned).toEqual([{ voice: 4, fxParams: { depth: 3 } }]),
+    },
+    {
+      name: 'voice 0 – nothing was played – reaches nothing, and the flow carries on',
+      inputs: { voice: 0, fxParams: { depth: 3 } },
+      next: 'exec',
+      after: (ctx) => expect(ctx.tuned).toEqual([]),
+    },
+  ],
   Log: [
     {
       name: 'logs its message and continues',
diff --git a/tests/core/soundfx.test.ts b/tests/core/soundfx.test.ts
index 180d02bd..8cf4383c 100644
--- a/tests/core/soundfx.test.ts
+++ b/tests/core/soundfx.test.ts
@@ -12,7 +12,14 @@ import type { BehaviorAsset, SoundFxAsset } from '../../src/core/model/Asset';
 import { isPlaceableType } from '../../src/core/model/AssetTypes';
 import { assetRefChoices } from '../../src/core/properties/assetRef';
 import { compileSoundFx, soundFxBuildFor } from '../../src/core/soundfx/compile';
-import { DRY, EMPTY_PLAN, type FxChain, fxChainOf, type FxPlan } from '../../src/core/soundfx/nodes';
+import {
+  DRY,
+  EMPTY_PLAN,
+  type FxChain,
+  fxChainOf,
+  type FxPlan,
+  type FxVoice,
+} from '../../src/core/soundfx/nodes';
 import { deserializeProject } from '../../src/core/store/serialize';
 import type { AudioSink } from '../../src/runtime/systems/Audio';
 import { paletteEntries } from '../../src/editor/graph/paletteEntries';
@@ -214,15 +221,15 @@ describe('a Mix in a sound FX graph (#316)', () => {
     expect(b).toMatchObject({ pitch: 0, stages: [{ kind: 'gain' }, { kind: 'filter' }] });
   });
 
-  it('takes its share from a parameter, held between 0 and 1', () => {
+  it('takes its share from a parameter, held between 0 and 1, and keeps both ways for it (#315)', () => {
     const f = forked();
     f.input.ports = [{ key: 'blend', kind: 'number', default: 0.25 }];
     f.edit.connect(f.input, 'blend', f.mix, 'mix');
     const plan = compileSoundFx(f.fx.graph).compiled!;
     expect(fxChainOf(plan)).toMatchObject({ mix: 0.25 });
     expect(fxChainOf(plan, { blend: 0.8 })).toMatchObject({ mix: 0.8 });
-    expect(fxChainOf(plan, { blend: -3 })).toMatchObject({ pitch: 12 });
-    expect(fxChainOf(plan, { blend: 3 })).toMatchObject({ pitch: 0 });
+    expect(fxChainOf(plan, { blend: -3 })).toMatchObject({ mix: 0, a: { pitch: 12 }, b: { pitch: 0 } });
+    expect(fxChainOf(plan, { blend: 3 })).toMatchObject({ mix: 1, a: { pitch: 12 }, b: { pitch: 0 } });
   });
 
   it('hands a Pitch behind it to both of its sounds', () => {
@@ -249,12 +256,20 @@ describe('a Mix in a sound FX graph (#316)', () => {
   });
 });
 
-/** Records what it was asked to play, with the line of effects. */
-function fakeAudio(): AudioSink & { played: { file: string; fx: FxChain | undefined }[] } {
+/**
+ * Records what it was asked to play, with the line of effects its values make, and every tune. A
+ * voice is numbered by how many have been played, so the first is 1.
+ */
+function fakeAudio(): AudioSink & {
+  played: { file: string; owner: string; fx: FxChain | undefined }[];
+  tuned: { voice: number; params: Readonly<Record<string, number>> }[];
+} {
   const sink = {
-    played: [] as { file: string; fx: FxChain | undefined }[],
-    play: (file: string, _volume: number, _loop: boolean, _owner: string, fx?: FxChain) =>
-      sink.played.push({ file, fx }),
+    played: [] as { file: string; owner: string; fx: FxChain | undefined }[],
+    tuned: [] as { voice: number; params: Readonly<Record<string, number>> }[],
+    play: (file: string, _volume: number, _loop: boolean, owner: string, fx?: FxVoice) =>
+      sink.played.push({ file, owner, fx: fx && fxChainOf(fx.plan, fx.params) }),
+    tune: (voice: number, params: Readonly<Record<string, number>>) => sink.tuned.push({ voice, params }),
     stop: () => {},
     stopOwner: () => {},
   };
@@ -286,7 +301,7 @@ describe('Play sound with fx', () => {
     const f = playingScene((s) => s.fx.id);
     const audio = fakeAudio();
     startWorld(f, { behaviors: true, audio }).frame();
-    expect(audio.played).toEqual([{ file: FILE, fx: { pitch: -12, stages: [] } }]);
+    expect(audio.played).toMatchObject([{ file: FILE, fx: { pitch: -12, stages: [] } }]);
   });
 
   it('plays it dry without an fx, with one that is gone, and with one that does not compile', () => {
@@ -305,7 +320,8 @@ describe('Play sound with fx', () => {
     expect(soundFxBuildFor(broken.fx).compiled).toBeNull();
     const brokenAudio = fakeAudio();
     startWorld(broken, { behaviors: true, audio: brokenAudio }).frame();
-    for (const audio of [none, gone, brokenAudio]) expect(audio.played).toEqual([{ file: FILE, fx: DRY }]);
+    for (const audio of [none, gone, brokenAudio])
+      expect(audio.played).toMatchObject([{ file: FILE, fx: DRY }]);
   });
 });
 
@@ -329,11 +345,11 @@ describe('the parameters of a sound FX', () => {
     expect(pitchOf(fxChainOf(plan, { other: 3 }))).toBe(-12);
   });
 
-  it('keep a stage that is 0 by default, so a value handed in can open it', () => {
+  it('keep a stage that is 0 by default, so a value handed in can open it, even on a playing voice', () => {
     const f = withParam('SoundFxGain', 'gain', [{ key: 'loud', kind: 'number' }]);
     const plan = soundFxBuildFor(f.fx).compiled!;
     expect(plan.stages).toHaveLength(1);
-    expect(fxChainOf(plan)).toEqual(DRY);
+    expect(fxChainOf(plan)).toEqual({ pitch: 0, stages: [{ kind: 'gain', gain: 1 }] });
     expect(fxChainOf(plan, { loud: 6 }).stages[0]).toMatchObject({ kind: 'gain' });
     expect((fxChainOf(plan, { loud: 6 }).stages[0] as { gain: number }).gain).toBeCloseTo(10 ** (6 / 20));
   });
@@ -437,7 +453,9 @@ describe('Sound FX parameters and Play sound', () => {
     const f = scene({ pick: ['depth'], values: { depth: 7 } });
     const audio = fakeAudio();
     startWorld(f, { behaviors: true, audio }).frame();
-    expect(audio.played).toEqual([{ file: FILE, fx: { pitch: 7, stages: [] } }]);
+    expect(audio.played).toMatchObject([
+      { file: FILE, fx: { pitch: 7, stages: [{ kind: 'gain', gain: 1 }] } },
+    ]);
   });
 
   it('starts a picked parameter at the default of the FX', () => {
@@ -449,7 +467,7 @@ describe('Sound FX parameters and Play sound', () => {
     ]);
     const audio = fakeAudio();
     startWorld(f, { behaviors: true, audio }).frame();
-    expect(audio.played[0]!.fx).toEqual({ pitch: -12, stages: [] });
+    expect(audio.played[0]!.fx).toEqual({ pitch: -12, stages: [{ kind: 'gain', gain: 1 }] });
   });
 
   it('says so at the node when no FX is picked or it is gone', () => {
@@ -475,9 +493,56 @@ describe('Sound FX parameters and Play sound', () => {
     expect(f.params.params.drop).toBe(7);
     const audio = fakeAudio();
     startWorld(f, { behaviors: true, audio }).frame();
-    expect(audio.played[0]!.fx).toEqual({ pitch: 7, stages: [] });
+    expect(audio.played[0]!.fx).toEqual({ pitch: 7, stages: [{ kind: 'gain', gain: 1 }] });
     f.stack.undo();
     expect(f.params.ports).toEqual([{ key: 'depth', kind: 'number' }]);
     expect(f.params.params.depth).toBe(7);
   });
 });
+
+describe('Set sound FX parameters (#315)', () => {
+  /**
+   * Pressing x plays Jump through an FX twice, the first `sound` – then hands `depth` to the voice
+   * the first play gave out.
+   */
+  function tuning(sound?: string) {
+    const f = makeSpriteScene('Box');
+    const { fx } = makeFx(f);
+    const created = new CreateSoundCommand('Jump', FILE);
+    f.stack.execute(created);
+    const behavior = makeBehavior(f, 'Logic');
+    place(f, behavior.id, { x: 0, y: 0 }, place(f, f.spriteId, { x: 0, y: 0 }));
+    const edit = graphEditor(f, behavior);
+    const first = edit.addNode('PlaySound', { sound: sound ?? created.asset.id, fx: fx.id });
+    const second = edit.addNode('PlaySound', { sound: created.asset.id, fx: fx.id });
+    const set = edit.addNode('SetSoundFxParameters');
+    const params = edit.addNode('SoundFxParameters', { soundFxAsset: fx.id, depth: 3 });
+    params.ports = [{ key: 'depth', kind: 'number' }];
+    edit.wire([
+      [edit.addNode('OnKeyDown', { key: 'x' }), 'exec', first, 'exec'],
+      [first, 'exec', second, 'exec'],
+      [second, 'exec', set, 'exec'],
+      [first, 'voice', set, 'voice'],
+      [params, 'params', set, 'fxParams'],
+    ]);
+    return f;
+  }
+
+  it('hands the values to the one voice its Play sound gave out, not to another of the same sound', () => {
+    const audio = fakeAudio();
+    const { input, frame } = startWorld(tuning(), { behaviors: true, audio });
+    input.press('x');
+    frame();
+    expect(audio.played).toHaveLength(2);
+    expect(audio.tuned).toEqual([{ voice: 1, params: { depth: 3 } }]);
+  });
+
+  it('reaches nothing where its Play sound played nothing', () => {
+    const audio = fakeAudio();
+    const { input, frame } = startWorld(tuning('no-such-sound'), { behaviors: true, audio });
+    input.press('x');
+    frame();
+    expect(audio.played).toHaveLength(1);
+    expect(audio.tuned).toEqual([]);
+  });
+});
diff --git a/tests/core/sounds.test.ts b/tests/core/sounds.test.ts
index 5a3dfe35..ae463302 100644
--- a/tests/core/sounds.test.ts
+++ b/tests/core/sounds.test.ts
@@ -42,6 +42,7 @@ function fakeAudio(): AudioSink & {
     stoppedOwners: [] as string[],
     play: (file: string, volume: number, loop: boolean, owner: string) =>
       sink.played.push({ file, volume, loop, owner }),
+    tune: () => {},
     stop: (file: string) => sink.stopped.push(file),
     stopOwner: (owner: string) => sink.stoppedOwners.push(owner),
   };
diff --git a/tests/helpers.ts b/tests/helpers.ts
index b376e514..d65527b9 100644
--- a/tests/helpers.ts
+++ b/tests/helpers.ts
@@ -214,6 +214,7 @@ export type FakeContext = NodeContext & {
     fxParams: Readonly<Record<string, number>>;
   }[];
   silenced: string[];
+  tuned: { voice: number; fxParams: Readonly<Record<string, number>> }[];
   /** The scenes the context was asked to load, in order. */
   loaded: string[];
   /** The time scale and the pause the context holds, as the game nodes left them. */
@@ -265,9 +266,12 @@ export function fakeContext(overrides: Partial<NodeContext> = {}): FakeContext {
     played: [],
     silenced: [],
     loaded: [],
+    // The number of a voice is how many have been played: unique, and never 0.
     playSound: (assetId, volume, loop, detached, fx, fxParams) =>
       ctx.played.push({ assetId, volume, loop, detached, fx, fxParams }),
     stopSound: (assetId) => ctx.silenced.push(assetId),
+    tuned: [],
+    tuneSound: (voice, fxParams) => ctx.tuned.push({ voice, fxParams }),
     loadScene: (sceneId) => ctx.loaded.push(sceneId),
     spawn: () => null,
     addJoint: () => null,
diff --git a/tests/viewport/sound-library.test.ts b/tests/viewport/sound-library.test.ts
index c88d2eaa..68e9ab7c 100644
--- a/tests/viewport/sound-library.test.ts
+++ b/tests/viewport/sound-library.test.ts
@@ -6,16 +6,28 @@
  * The device is stubbed – nothing makes a noise in a unit test, and Web Audio does not exist here.
  */
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
-import type { FxChain } from '../../src/core/soundfx/nodes';
+import type { FxChain, FxPlan, FxVoice } from '../../src/core/soundfx/nodes';
 import { SoundLibrary } from '../../src/viewport/SoundLibrary';
 
 const FILE = 'a.wav';
 const OTHER = 'b.wav';
 
+/** An AudioParam that remembers where it was sent to glide, and how often. */
+interface FakeParam {
+  value: number;
+  glides: number[];
+  setTargetAtTime: (value: number) => void;
+}
+
+function param(value: number): FakeParam {
+  const made: FakeParam = { value, glides: [], setTargetAtTime: (to) => made.glides.push(to) };
+  return made;
+}
+
 interface FakeSource {
   buffer: unknown;
   loop: boolean;
-  playbackRate: { value: number };
+  playbackRate: FakeParam;
   onended: (() => void) | null;
   started: boolean;
   stopped: boolean;
@@ -28,11 +40,11 @@ const sources: FakeSource[] = [];
 /** Every `connect` made, as "from > to" by the kind of node – the order a voice is wired in. */
 const wires: string[] = [];
 /** The level of every gain node made, in the order they were made. */
-const gains: { value: number }[] = [];
+const gains: FakeParam[] = [];
 const filters: {
   type: string;
   frequency: { value: number };
-  gain: { value: number };
+  gain: FakeParam;
   Q: { value: number };
 }[] = [];
 
@@ -69,6 +81,7 @@ function fakeBuffer(_channels: number, length: number, sampleRate: number) {
 function fakeContext() {
   return {
     state: 'running',
+    currentTime: 0,
     destination: { kind: 'destination' },
     resume: () => Promise.resolve(),
     suspend: () => Promise.resolve(),
@@ -78,7 +91,7 @@ function fakeContext() {
       const source: FakeSource = wired('source', {
         buffer: null,
         loop: false,
-        playbackRate: { value: 1 },
+        playbackRate: param(1),
         onended: null,
         started: false,
         stopped: false,
@@ -93,7 +106,7 @@ function fakeContext() {
       return source;
     },
     createGain: () => {
-      const gain = wired('gain', { gain: { value: 1 } });
+      const gain = wired('gain', { gain: param(1) });
       gains.push(gain.gain);
       return gain;
     },
@@ -101,7 +114,7 @@ function fakeContext() {
       const filter = wired('filter', {
         type: '',
         frequency: { value: 0 },
-        gain: { value: 0 },
+        gain: param(0),
         Q: { value: 0 },
       });
       filters.push(filter);
@@ -136,11 +149,37 @@ afterEach(() => {
   vi.unstubAllGlobals();
 });
 
-/** Plays `file` and lets its bytes arrive, unless `hold` keeps the decode pending. */
-async function play(library: SoundLibrary, file = FILE, { hold = false } = {}, fx?: FxChain): Promise<void> {
-  library.play(file, 1, false, '', fx);
+/** A chain as the plan of an FX with nothing left open, so a test can say what it wants to hear. */
+function planOf(chain: FxChain): FxPlan {
+  const stages = chain.stages.map((stage) =>
+    stage.kind === 'gain'
+      ? { kind: 'gain' as const, db: { value: 20 * Math.log10(stage.gain) } }
+      : { ...stage, gain: { value: stage.gain } },
+  );
+  if ('mix' in chain) return { mix: { value: chain.mix }, a: planOf(chain.a), b: planOf(chain.b), stages };
+  return { pitch: chain.pitch === 0 ? [] : [{ value: chain.pitch }], stages };
+}
+
+/** Plays `file` and lets its bytes arrive, unless `hold` keeps the decode pending; gives the voice. */
+async function play(
+  library: SoundLibrary,
+  file = FILE,
+  { hold = false, owner = '' } = {},
+  fx?: FxChain | FxVoice,
+): Promise<number> {
+  const voice = library.play(
+    file,
+    1,
+    false,
+    owner,
+    fx && ('plan' in fx ? fx : { plan: planOf(fx), params: {} }),
+  );
   await settle();
-  if (hold) return;
+  if (!hold) await arrive();
+  return voice;
+}
+
+async function arrive(): Promise<void> {
   for (const resolve of decodes.splice(0)) resolve();
   await settle();
 }
@@ -208,7 +247,7 @@ describe('the sound library', () => {
     expect([up!.duration, down!.duration]).toEqual([dry!.duration, dry!.duration]);
   });
 
-  it('mixes two sounds by their shares, a source per pitch, started and stopped together (#316)', async () => {
+  it('mixes two sounds by their shares, a source per way, started and stopped together (#316)', async () => {
     const library = new SoundLibrary();
     await play(
       library,
@@ -231,7 +270,7 @@ describe('the sound library', () => {
       'filter > gain',
       'gain > destination',
     ]);
-    expect(gains.map((g) => g.value)).toEqual([1, 1, 0.75, 0.5, 0.25]);
+    expect(gains.map((g) => g.value)).toEqual([1, 1, 0.75, expect.closeTo(0.5), 0.25]);
     expect(sources.map((s) => s.started)).toEqual([true, true]);
     expect(library.stats()).toMatchObject({ playing: 1, started: 1, last: { pitches: [12, 0], stages: 2 } });
 
@@ -242,7 +281,7 @@ describe('the sound library', () => {
     expect(library.stats()).toMatchObject({ playing: 0, stopped: 1 });
   });
 
-  it('starts one source for two ways at the same pitch', async () => {
+  it('gives two ways at the same pitch a source each, so each can glide on its own (#315)', async () => {
     const library = new SoundLibrary();
     await play(
       library,
@@ -255,10 +294,82 @@ describe('the sound library', () => {
         stages: [],
       },
     );
-    expect(sources).toHaveLength(1);
+    expect(sources).toHaveLength(2);
+    expect(sources[0]!.buffer).toBe(sources[1]!.buffer);
     expect(wires.filter((w) => w.startsWith('source'))).toEqual(['source > gain', 'source > gain']);
   });
 
+  describe('a parameter changed while the voice plays (#315)', () => {
+    /** Pitch on a, a gain on b, the share between them – all three open – and a band after the Mix. */
+    const open: FxPlan = {
+      mix: { value: 0.25, param: 'blend', min: 0, max: 1 },
+      a: { pitch: [{ value: 0, param: 'depth', min: -64, max: 64 }], stages: [] },
+      b: { pitch: [], stages: [{ kind: 'gain', db: { value: 0, param: 'loud', min: -120, max: 24 } }] },
+      stages: [{ kind: 'filter', filter: 'lowshelf', frequency: 80, gain: { value: 6 }, q: 1 }],
+    };
+
+    it('glides every knob it moves, a pitch by speed, and leaves the rest alone', async () => {
+      const library = new SoundLibrary();
+      const voice = await play(library, FILE, { owner: 'me' }, { plan: open, params: {} });
+      // Made in the order: the voice, the Mix, share of a, the gain on b, share of b.
+      const [, , shareA, loud, shareB] = gains;
+      expect([shareA!.value, loud!.value, shareB!.value]).toEqual([0.75, 1, 0.25]);
+
+      expect(library.tune(voice, { depth: 12, loud: -6, blend: 0.5, nothing: 3 })).toBe(true);
+      expect(sources[0]!.playbackRate.glides).toEqual([2]);
+      expect(sources[1]!.playbackRate.glides).toEqual([]);
+      expect(loud!.glides).toEqual([expect.closeTo(10 ** (-6 / 20))]);
+      expect([shareA!.glides, shareB!.glides]).toEqual([[0.5], [0.5]]);
+      expect(filters[0]!.gain.glides).toEqual([]);
+      expect(library.stats().tuned).toBe(1);
+
+      // Only what changed is sent again; one not given keeps its value rather than its default.
+      library.tune(voice, { depth: 12, blend: 1 });
+      expect(sources[0]!.playbackRate.glides).toEqual([2]);
+      expect(loud!.glides).toHaveLength(1);
+      expect([shareA!.glides, shareB!.glides]).toEqual([
+        [0.5, 0],
+        [0.5, 1],
+      ]);
+    });
+
+    it('reaches the one voice it is given – a detached one too – and none that has ended', async () => {
+      const library = new SoundLibrary();
+      const voices = [
+        await play(library, FILE, { owner: 'me' }, { plan: open, params: {} }),
+        await play(library, FILE, { owner: 'me' }, { plan: open, params: {} }),
+        await play(library, OTHER, { owner: 'me' }, { plan: open, params: {} }),
+        await play(library, FILE, {}, { plan: open, params: {} }),
+      ];
+      expect(new Set(voices).size).toBe(4);
+      expect(voices).not.toContain(0);
+      library.tune(voices[1]!, { depth: 12 });
+      library.tune(voices[3]!, { depth: 12 });
+      // Two sources a voice, the first of each the pitched way.
+      expect(sources.map((s) => s.playbackRate.glides.length)).toEqual([0, 0, 1, 0, 0, 0, 1, 0]);
+
+      library.stop(FILE);
+      expect(library.tune(voices[1]!, { depth: 3 })).toBe(false);
+      expect(library.tune(0, { depth: 3 })).toBe(false);
+      expect(library.stats().tuned).toBe(2);
+    });
+
+    it('starts a voice still waiting on its bytes with the values handed in meanwhile', async () => {
+      const library = new SoundLibrary();
+      const voice = await play(
+        library,
+        FILE,
+        { hold: true, owner: 'me' },
+        { plan: open, params: { blend: 0.5 } },
+      );
+      expect(library.tune(voice, { depth: 7 })).toBe(true);
+      await arrive();
+      expect(library.stats().last?.pitches).toEqual([7, 0]);
+      expect(gains.map((g) => g.value)).toContain(0.5);
+      expect(sources[0]!.playbackRate.glides).toEqual([]);
+    });
+  });
+
   it('a stop while the bytes are in flight keeps the sound from starting afterwards', async () => {
     const library = new SoundLibrary();
     await play(library, FILE, { hold: true });
diff --git a/tools/e2e/34-sound.mjs b/tools/e2e/34-sound.mjs
index 56d03ed0..3a635d9f 100644
--- a/tools/e2e/34-sound.mjs
+++ b/tools/e2e/34-sound.mjs
@@ -1,4 +1,4 @@
-import { edge, node } from '../lib/builders.mjs';
+import { edge, node, port, variable, written } from '../lib/builders.mjs';
 import { LANES, MAKE_OBJECT, pickAssetType } from './lib/scaffold.mjs';
 
 /**
@@ -414,6 +414,72 @@ export async function sound(t) {
     JSON.stringify(gameParam),
   );
   await stop();
+  // #315: two looping voices of the same sound start at depth 7, the first keeps its number in a
+  //       variable, and `Set sound FX parameters` hands that one voice depth 3 every frame. In a
+  //       real AudioContext the first one's speed glides there, the second one's stays put.
+  const tuneIds = ['sn-keep', 'sn-play2', 'sn-tick', 'sn-read', 'sn-tune', 'sn-fxp2'];
+  const tuneEdges = [
+    'sn-e-keep',
+    'sn-e-voice',
+    'sn-e-play2',
+    'sn-e-fxp3',
+    'sn-e-tick',
+    'sn-e-read',
+    'sn-e-fxp2',
+  ];
+  await mutate(`
+    const logic = __gm.state.project.assets.find((a) => a.id === 'sn-logic');
+    logic.variables = [${JSON.stringify(variable('voice', 'number', 0))}];
+    const graph = logic.graph;
+    const play = graph.nodes.find((n) => n.id === 'sn-play');
+    play.params.loop = 'true';
+    graph.nodes.push(
+      ${JSON.stringify(node('sn-keep', 'Variables', 520, 0, {}, [written('voice', 'number')]))},
+      { ...structuredClone(play), id: 'sn-play2', x: 780 },
+      ${JSON.stringify(node('sn-tick', 'OnUpdate', 0, 300))},
+      ${JSON.stringify(node('sn-read', 'Variables', 0, 420, {}, [port('voice', 'number')]))},
+      ${JSON.stringify(node('sn-tune', 'SetSoundFxParameters', 260, 300))},
+      ${JSON.stringify(
+        node('sn-fxp2', 'SoundFxParameters', 0, 540, { soundFxAsset: fxId, depth: 3 }, [
+          { key: 'depth', kind: 'number' },
+        ]),
+      )},
+    );
+    graph.edges.push(
+      ${JSON.stringify(edge('sn-e-keep', 'sn-play', 'exec', 'sn-keep', 'exec'))},
+      ${JSON.stringify(edge('sn-e-voice', 'sn-play', 'voice', 'sn-keep', 'voice'))},
+      ${JSON.stringify(edge('sn-e-play2', 'sn-keep', 'exec', 'sn-play2', 'exec'))},
+      ${JSON.stringify(edge('sn-e-fxp3', 'sn-fxp', 'params', 'sn-play2', 'fxParams'))},
+      ${JSON.stringify(edge('sn-e-tick', 'sn-tick', 'exec', 'sn-tune', 'exec'))},
+      ${JSON.stringify(edge('sn-e-read', 'sn-read', 'voice', 'sn-tune', 'voice'))},
+      ${JSON.stringify(edge('sn-e-fxp2', 'sn-fxp2', 'params', 'sn-tune', 'fxParams'))},
+    );`);
+  const beforeTune = await stats();
+  await play();
+  const tunedStats = await waitFor(
+    stats,
+    (s) => s.tuned > beforeTune.tuned && s.started >= beforeTune.started + 2,
+  );
+  const rate = 2 ** ((3 - 7) / 12);
+  const glided = (levels) =>
+    Array.isArray(levels) && levels.length === 2 && Math.abs(levels[0][0] - rate) < 0.01;
+  const levels = await waitFor(() => json(`__gm.sound.levels(${JSON.stringify(blip.file)})`), glided);
+  check(
+    'Set sound FX parameters moves the one voice whose number it holds from depth 7 to 3, not the other',
+    tunedStats.last?.pitches.join() === '7' &&
+      tunedStats.last.loop === true &&
+      glided(levels) &&
+      Math.abs(levels[0][1] - 10 ** (-6 / 20)) < 0.001 &&
+      levels[1][0] === 1,
+    JSON.stringify({ last: tunedStats.last, tuned: tunedStats.tuned, levels, rate }),
+  );
+  await stop();
+  await mutate(`
+    const logic = __gm.state.project.assets.find((a) => a.id === 'sn-logic');
+    logic.variables = [];
+    const graph = logic.graph;
+    graph.nodes = graph.nodes.filter((n) => !${JSON.stringify(tuneIds)}.includes(n.id));
+    graph.edges = graph.edges.filter((e) => !${JSON.stringify(tuneEdges)}.includes(e.id));`);
   await mutate(`
     const graph = __gm.state.project.assets.find((a) => a.id === 'sn-logic').graph;
     graph.nodes = graph.nodes.filter((n) => n.id !== 'sn-fxp');