My setup is Sony XBR series running Android TV, and a 7.1 capable sound bar.
Plex has no issues with playback on sub 7.1 stream, but on AAC 7.1 it is silent. This seems to be isolated to the Android TV Plex app as the following works:
Native Sony/Video Playback app: works
Kodi for Android TV: works
Plex for XBOX One (connected to The same TV/sound system) : works
The only one that is flaky is the Plex Android TV app. Would prefer not to have to fire up the Xbox to watch Plex and the other uPNP browsers are weak visually.