I spent way too much time troubleshooting BG3 on my Steam Deck because it would not start, and when it finally did it had no sound.
The fix at least for me was quite simple, and someone mentioned it on a reddit post lost in the middle on nowhere.
Fix: Uninstall Decky’s PowerTools.
After I removed it, the game just simply worked.
That’s my best guess, maybe I changed it for some game and forgot about it.