diff --git a/src/components/views/rooms/VoiceRecordComposerTile.tsx b/src/components/views/rooms/VoiceRecordComposerTile.tsx
index 5d984eacfa..709eab82a0 100644
--- a/src/components/views/rooms/VoiceRecordComposerTile.tsx
+++ b/src/components/views/rooms/VoiceRecordComposerTile.tsx
@@ -95,7 +95,7 @@ export default class VoiceRecordComposerTile extends React.PureComponent<IProps,
                 duration: Math.round(this.state.recorder.durationSeconds * 1000),
 
                 // https://github.com/matrix-org/matrix-doc/pull/3246
-                waveform: this.state.recorder.getPlayback().waveform.map(v => Math.round(v * 1024)),
+                waveform: this.state.recorder.getPlayback().thumbnailWaveform.map(v => Math.round(v * 1024)),
             },
             "org.matrix.msc3245.voice": {}, // No content, this is a rendering hint
         });
diff --git a/src/voice/Playback.ts b/src/voice/Playback.ts
index 6a120bf924..1a1ee54466 100644
--- a/src/voice/Playback.ts
+++ b/src/voice/Playback.ts
@@ -31,6 +31,7 @@ export enum PlaybackState {
 }
 
 export const PLAYBACK_WAVEFORM_SAMPLES = 39;
+const THUMBNAIL_WAVEFORM_SAMPLES = 100; // arbitrary: [30,120]
 const DEFAULT_WAVEFORM = arraySeed(0, PLAYBACK_WAVEFORM_SAMPLES);
 
 function makePlaybackWaveform(input: number[]): number[] {
@@ -51,6 +52,12 @@ function makePlaybackWaveform(input: number[]): number[] {
 }
 
 export class Playback extends EventEmitter implements IDestroyable {
+    /**
+     * Stable waveform for representing a thumbnail of the media. Values are
+     * guaranteed to be between zero and one, inclusive.
+     */
+    public readonly thumbnailWaveform: number[];
+
     private readonly context: AudioContext;
     private source: AudioBufferSourceNode;
     private state = PlaybackState.Decoding;
@@ -72,6 +79,7 @@ export class Playback extends EventEmitter implements IDestroyable {
         this.fileSize = this.buf.byteLength;
         this.context = createAudioContext();
         this.resampledWaveform = arrayFastResample(seedWaveform ?? DEFAULT_WAVEFORM, PLAYBACK_WAVEFORM_SAMPLES);
+        this.thumbnailWaveform = arrayFastResample(seedWaveform ?? DEFAULT_WAVEFORM, THUMBNAIL_WAVEFORM_SAMPLES);
         this.waveformObservable.update(this.resampledWaveform);
         this.clock = new PlaybackClock(this.context);
     }