When the latest segment lists stems (audio tracks in groups above the full mix's), the player menu gets an Audio section: the full mix by default, or each stem at Off/25/50/100%. WHEP offers three audio transceivers (full mix, then up to two stems). On web the video element plays the video and the full mix, and each stem mixed in plays from its own audio element; on native each track gets its own _setVolume. Track events from unused transceivers carry no stream and no longer replace the player's stream. Covered by flows/audio-stems.spec.ts and .maestro/logged-out/audio-stems.yaml, which run when the harness pushes over fMP4.