I think there's miscommunication. I don't mean you're audio is out of sync, I mean that the file being played has 7.1 as the audio track, then when jellyfin downmixes it to stereo for you speakers it sounds like absolute ass.
The "doesn't select the right audio track" is another issue that I personally don't have, but the downmixing one is something that seems to be baked into Jellyfin for TV.
Not a bad idea honestly. That's my next project after a home server upgrade.