Here’s what I tested:
PMS version & platform: multiple tested, does not matter
Plex Android Client: 9.16.0.38368, Pixel 6a, Android 13, on WiFi, same subnet as Plex server.
Given the file structure below, the Plex Android mobile client will not play Movie2, with EAC3 audio. The user sees a spinning orange circle. Manually selecting the version does work.
Movie1, with AC3 audio, plays immediately.
The Plex Android mobile app on a Pixel 6a direct plays AC3 audio. EAC3 audio is transcoded by Plex Media Server to a supported format.
/Movies
├───Movie1 (year)
│ Movie1 (year) [1080p.AC3].mkv
│ Movie1 (year) [720p.AC3].mkv
│
└───Movie2 (year)
Movie2 (year) [1080p.EAC3].mkv
Movie2 (year) [720p.EAC3].mkv
