No 5.1 Surround Sound for PS4 Plex app?

I recently downloaded the Plex Plus app for the PS4. I get 5.1 surround sound while playing games on the PS4 (Diablo 3, for example, sounds fine), as does Netflix movies, but when I play a movie or show via the PS4's Plex app, sound only comes out of the 2 side speakers. 

 

Is this an issue with the encoding of the files, or is there a setting somewhere in Plex that I need to change? For reference, most of the video files I try are x264 video (mp4) and AAC audio. 

Have you tried the settings in the overlay once the content is playing?