I just went through this also.
there are a only a few thing you can do and neither are very good.
If your android box is like mine the only real 100% fix was to root the box and edit a Plex file to disable AC3.
In my case i could get it to work on my box by turning on HDMI Pass through in Plex and then in android sound setting chose manual and select ac3 and aac only. This would get me sound in Plex but after i ran kodi and switched back to plex sound was gone again. I had to either force close Plex and restart or go back to android sound and switch from manual to auto and then back to manual. This was a Pain in the ass.
So i have disabled AC3 by editing like so
C/P
Was able to fix this bug on a rooted H96Pro +
the file
/data/data/com.plexapp.android/shared_prefs/com.plexapp.android.xml
needs to be modified
<boolean name=“general.deviceSupportsDTS” value=“ false ”>
<boolean name=" general.deviceSupportsAC3" value=“ false ”>
