When receiving 5.1 channel Opus content over WebRTC, all channels are received and decoded but downmixed to stereo, even when a 5.1-compatible sound card is connected. <audio/> tags, on the other hand, do render 5.1 content properly, e.g., https://www2.iis.fraunhofer.de/AAC/multichannel.html. This suggests that somewhere along the line a mixer is not being initialized with the right number of channels. I looked through the playout path and didn't see anything obvious, but I didn't investigate closely.
<rdar://problem/70408614>