🐛(frontend) omit the default speaker id sent to LiveKit

Firefox rejects setSinkId('default') when a remote audio track
is attached. Omit that id and only send a real device id.

CHANGELOG.md

@@ -26,6 +26,7 @@ and this project adheres to
 - 🔒️(frontend) upgrade tiff to fix CVE-2026-4775
 - 🔒️(backend) bind accepted lobby entries to the current username
 - 🔒️(backend) sign lobby participant id
+- 🐛(frontend) omit the default speaker id sent to LiveKit
 
 ## [1.34.0] - 2026-10-07
 

src/frontend/src/features/rooms/components/Conference.tsx

@@ -45,6 +45,7 @@ import { userStore } from '@/stores/user'
 import { WatchMediaDeviceErrors } from './WatchMediaDeviceErrors'
 import { MeetDevtools } from '@/features/devtools'
 import { VOICE_AUDIO_CONSTRAINTS } from '@/features/rooms/livekit/utils/constants'
+import { resolveAudioOutputDeviceId } from '@/features/rooms/utils/resolveAudioOutputDeviceId'
 
 export const Conference = ({
   roomId,
@@ -120,7 +121,7 @@ export const Conference = ({
         ...VOICE_AUDIO_CONSTRAINTS,
       },
       audioOutput: {
-        deviceId: userConfig.audioOutputDeviceId ?? undefined,
+        deviceId: resolveAudioOutputDeviceId(userConfig.audioOutputDeviceId),
       },
     }
     // do not rely on the userConfig object directly as its reference may change on every render

src/frontend/src/features/rooms/utils/resolveAudioOutputDeviceId.ts

@@ -0,0 +1,11 @@
+/**
+ * `'default'` means "use the system output". Firefox has no device with that
+ * id, so LiveKit's setSinkId('default') rejects on every remote audio attach.
+ * Omit it and the browser keeps the OS default, which is the same result.
+ */
+export const resolveAudioOutputDeviceId = (
+  deviceId: string | undefined
+): string | undefined => {
+  if (!deviceId || deviceId === 'default') return undefined
+  return deviceId
+}