Longstanding feature request. Add your vote!
There is a partial work around for Plex Web & Plex for Windows/Mac. Unsure if this works for other clients. It does not for Android mobile.
In Plex Web and Plex for Windows/Mac, Plex displays the Name field of the audio track. Add “TrueHD + Atmos” etc to the field and Plex will display it. This only works if there are multiple audio tracks in the file. Plex only displays the name in the drop down menu, and there is no menu if there is only one audio track.
You can modify the Name field of MKV files with MKVToolNix.
Plex Web:
MKVToolNix Header Editor
