Hello and happy new year.
What a big good surprise. Android 9 (Pie) is available since yesterday for the Mibox S
This new release adds new audio features. Now it’s possible to set manually the audio capabilities of your system. With 8.0 only automatic settings was available. So I set it manually and now Plex app is able to play AC3 and DTS audio files correctly !
What does mean ? It means that what @anon18523487 said is not really the reality. This is not Plex app which detects HDMI capabilities. Plex app asks Android system for the audio capabilities which is not the same story. So if Android system is not able to catch from HDMI the right configuration, it will send this wrong configuration to others apps which ask for.
Why Kodi had not this issue with Android 8.0 release ? Kodi doesn’t ask the system for the audio capabilities, you can force it directly to its settings.
I hope that all this …long story will help others 
