Comment by englishm

Synchronized playback is usually primarily a player responsibility, not something you should (solely) rely on your transport to provide. We have had some talk about extensions to allow for synchronizing multiple tracks by group boundaries at each hop through a relay system, but it's not clear if that's really needed yet.

Essentially though, there are typically some small jitter buffers at the receiver and the player knows how draw from those buffers, syncing audio and video. Someone who works more on the player side could probably go into a lot more interesting detail about approaches to doing that, especially at low latencies. I know it can also get complicated with hardware details of how long it takes an audio sample vs. a video frame to actually be reproduced once the application sinks it into the playback queue.