Plex Plays movies that some speakers and sound systems don't have the correct codec to interpret.
Do this to fix it:
Play the movie then click the center button to bring up the playback line, then click on HD and choose a different internet speed. Plex will then encode the video in a playback audio will work on you speakers, If you find that too complicated, Ask to switch to LimePie which is similar to plex, but plays audio that works on any sound system.