For those new to this thread and those still confused, let me try to summarize what is going on.
There have been two issues since stable version 8.30.2.31202. The first one, an audio/video sync problem with TrueHD/Atmos, cropped up in 8.31.0.31102-beta, courtesy of
FIXES:
- Player: improve TrueHD playback performance.
This issue was supposed to have been fixed in stable release 8.31.1.31487, as noted by
FIXES:
- Player: ensure TrueHD remains in sync.
The issue persisted, though, through another stable version, 8.31.2.31546, which was the latest available until the 9.0.0.31676 release this morning.
Which brings us to the 9.x.x beta releases and the second issue being discussed here. A fix for the audio/video sync problem was put into 9.0.0.31509-beta, although nothing particular was mentioned in the release notes.
That same version introduced a new issue, which prevented a lot of files with TrueHD/Atmos from playing at all. 9.0.0.31650-beta tried to revert something that might have caused this behavior, but the issue remained.
BETA:
- Player: remove TrueHD timestamp precision changes.
@sixones has since given us two experimental builds to test, 9.1.0.31664 and 9.1.0.31687, which both include a new demuxer. And that seems to now have fixed the second issue with lots of files simply not starting.
What a mess 
Let’s please make this experimental build the official beta as quickly as possible! Thanks to all who have been contributing to this thread.
EDIT on 3/31/2022: to add to this, today Plex released stable 9.0.0.31676, which does not seem to contain the fix for the playback issue. My question is, why was this released then? Probably just to fix the audio/video sync issue, but that doesn’t make sense when the playback issue is arguably the bigger one.
EDIT on 4/1/2022: Today, stable version 9.0.1.31729 was released, but does not appear to have the fix for the playback issue.
Also released today was 9.1.0.31720-beta, which does fix the playback issue. This should be the version with the fewest issues, currently.
