I have a 7.1 surround system, and Plex (PS4), whenever playing any audio, outputs audio to all 7.1 speakers all the time. Be it stereo, or 5.1.
This means when playing 5.1 surround sound, the back speakers simply output no audio.
For any other 5.1 audio, my receiver matrixes the 5.1 audio to 7.1 with Dolby Pro Logic so it has proper location. Because Plex doesn't do this I can't.
Plex seems to be the only PS4 app that does this, others will switch to 5.1 for 5.1 audio.
@mattseeley have you looked into whether this is possible? I notice other apps will adjust their channels accordingly (like Netflix, Vudu, etc). Oddly enough the native Media Player doesn’t do it, along with Plex.