Bug 320087 – iOS Safari: `setSinkId()` returns success but audio output doesn't actually switch on single-track WebRTC MediaStream. Works correctly with dual-track streams.

• Status: RESOLVED
• Resolution: FIXED
• Component: WebRTC

[ernst.zhu]

# iOS Safari WebRTC Speaker Switch Bug Report

## Bug Summary
`setSinkId()` returns success but audio output doesn't actually switch on single-track WebRTC MediaStream. Works correctly with dual-track streams.

## Reproducible Test Case
Test page: `https://okmimi521.github.io/Media/ios_webrtc_speaker_test/`

## Detailed Description

### Problem
On iOS Safari, when an `HTMLAudioElement` receives a remote MediaStream via WebRTC containing only ONE audio track, calling `setSinkId()` returns a resolved Promise (appears successful) but the audio output device does NOT actually switch. However, with TWO or more audio tracks in the same WebRTC stream, `setSinkId()` works correctly and audio actually routes to the selected device.

### Technical Details

**Failing Scenario (Silent Failure):**
```javascript
// Single track from WebRTC
remotePeer.ontrack = (event) => {
  const stream = event.streams[0]; // MediaStream with 1 audio track
  audioElement.srcObject = stream;
  
  // Promise resolves but audio doesn't switch:
  await audioElement.setSinkId(deviceId); // ✅ Promise resolves
  console.log(audioElement.sinkId); // Shows new deviceId
  // ❌ But audio still plays from old device!
};
```

**Working Scenario:**
```javascript
// Dual track from WebRTC
remotePeer.ontrack = (event) => {
  const stream = event.streams[0]; // MediaStream with 2 audio tracks
  audioElement.srcObject = stream;
  
  // Promise resolves AND audio actually switches:
  await audioElement.setSinkId(deviceId); // ✅ Promise resolves
  console.log(audioElement.sinkId); // Shows new deviceId
  // ✅ Audio correctly plays from new device!
};
```

### Key Observations

1. **Silent failure**: `setSinkId()` Promise resolves successfully, `audioElement.sinkId` property updates to the new deviceId, but the actual audio routing does NOT change
2. **Only affects WebRTC streams**: Local MediaStreams from `getUserMedia()` or file-based streams work fine with single track
3. **No console errors**: The bug is completely silent - no exceptions, no warnings

### Steps to Reproduce

1. Open test page on iOS Safari: `https://localhost/ios_webrtc_speaker_test/index.html`
2. Click **"Send SINGLE Track"** button
   - Creates a WebRTC peer connection
   - Loads one audio file and creates MediaStream with 1 track
   - Transmits via `RTCPeerConnection.addTrack()`
   - Receiver gets stream via `ontrack` event
3. Wait for connection to establish
4. Click **"🎤 Get Microphone Permission"** (optional, for full device labels)
5. Click **"Refresh Speakers"**
6. Select a different speaker from dropdown
7. **Observe**: 
   - Console shows: `✓ setSinkId() SUCCEEDED`
   - `audioElement.sinkId` shows the new deviceId
   - **BUT audio still plays from the original device** (no actual device switch)
   - Put device near ear to verify audio is still from earpiece/speaker/headphones
8. Click **"Send DUAL Track"** button
   - Same process but with 2 audio tracks
9. Refresh speakers and select different device
10. **Observe**: 
    - Console shows: `✓ setSinkId() SUCCEEDED`
    - `audioElement.sinkId` shows the new deviceId
    - **Audio actually switches to the selected device** ✓

### Expected Result
`setSinkId()` should successfully change the audio output device regardless of the number of audio tracks in the WebRTC MediaStream. Both the Promise resolution and the actual audio routing should succeed.

### Actual Result
- **Single track WebRTC stream**: 
  - ✅ `setSinkId()` Promise resolves
  - ✅ `audioElement.sinkId` property updates
  - ❌ Audio continues playing from the ORIGINAL device (no actual device switch)
- **Dual track WebRTC stream**: 
  - ✅ `setSinkId()` Promise resolves
  - ✅ `audioElement.sinkId` property updates
  - ✅ Audio correctly plays from the NEW device

### Environment
- **Device**: iPhone 15
- **iOS Version**: iOS 26.5
- **WebRTC**: Standard RTCPeerConnection API

### Impact
- **Severity**: High (Silent Failure - No Error Indication)
- **Affected Users**: Any iOS Safari users in WebRTC applications trying to switch audio output devices (e.g., video conferencing apps, VoIP apps)
- **User Experience**: Users THINK they switched devices (UI shows success) but audio doesn't actually change. This is worse than an error because users don't know something is wrong.
- **Real-world scenario**: User tries to switch from earpiece to speakerphone during a WebRTC call, sees "success", but audio still plays from earpiece. Very confusing UX.

### Related APIs
- `HTMLMediaElement.setSinkId()`
- `RTCPeerConnection`
- `MediaStream`
- `MediaStreamTrack`

### Console Output
**No errors are thrown!** This is a silent bug:

```javascript
// Single track - appears successful but doesn't work
await audioElement.setSinkId('speaker-device-id');
console.log(audioElement.sinkId); // Output: "speaker-device-id" ✅
// But audio still plays from earpiece ❌

// Dual track - actually works
await audioElement.setSinkId('speaker-device-id');
console.log(audioElement.sinkId); // Output: "speaker-device-id" ✅
// Audio correctly plays from speaker ✅
```

**Testing Method**:
1. Switch from earpiece to speaker
2. Put iPhone near your ear
3. If you can still hear clearly = bug present (audio didn't switch)
4. If audio is faint/distant = working correctly (audio switched to speaker)

### Additional Testing
Tested with:
- ✅ File → MediaStream → Audio element (single track): **Works**
- ✅ getUserMedia() → Audio element (single track): **Works**
- ❌ WebRTC → Audio element (single track): **Fails**
- ✅ WebRTC → Audio element (dual track): **Works**

This isolates the issue to WebRTC MediaStreams with single track only.

---

## For Apple Engineers

### Suspected Root Cause
Likely an issue in WebKit's audio routing logic that assumes WebRTC streams should have multiple tracks, or a bug in the iOS audio session management when handling single-track remote streams.

### Suggested Investigation Areas
1. `WebCore/Modules/webaudio/MediaStreamAudioDestinationNode`
2. `Source/WebCore/platform/audio/ios/AudioDestinationIOS`
3. `RTCAudioSession` handling in WebKit
4. Audio routing logic in `HTMLMediaElement::setSinkId()` for remote tracks

### Test Automation
This bug should be reproducible in WebKit layout tests with:
- Create local RTCPeerConnection loopback
- Single track vs dual track scenarios
- Call setSinkId() on receiver side
- Assert device change succeeds

---

**Date**: 2026-07-23
**Reported by**: Zoom
**Contact**: [e-mail]

[ernst.zhu]

open

[Alexey Proskuryakov]

*** Bug 320086 has been marked as a duplicate of this bug. ***

[Radar WebKit Bug Importer]

<rdar://problem/183147942>

[youenn fablet]

Pull request: https://github.com/WebKit/WebKit/pull/70428

[EWS]

Committed 318149@main (bdaffe35a57e): <https://commits.webkit.org/318149@main>

Reviewed commits have been landed. Closing PR #70428 and removing active labels.

[ernst.zhu]

Hi everyone. I noticed that this bug has been fixed. Could you please let me know in which version of Safari it will be released? Thanks!!!