Well, the problem has gone away. I’m now on 6.10.12-1. I do notice that lots of things are different now. Not only does the sound work but my Compose key is working again and my lock screen has the right theme (before it was kind of generic). I wonder if it was not some error with how the session was created. Perhaps when trying to connect to the old session, it failed and created a new one, but the old one was still there. I have no clue, honestly. Sorry for troubling you.