In the 8.14 beta, there are some potential improvements for the audio/video sync that are currently “flagged-off”, which I can enable for people to test and report back if they’d like to try it out. If you want to try it, DM me your Plex username, and I’ll enable it for you. It will require killing and restarting the app after I enable it, but once it’s done, you should see some improvements.
