This happens when you try to play a movie or show that is 5.1 surround sound or 7.1 but don't have a way to play it.
2 ways to fix this.
First way is to Go and click on the HD underneath the Playback line during playback. Then Click More. Then Choose 720p 3mbs. This will fix most sound issues in Plex.
Also Go to Settings on the home screen of Plex. you will see this on the far bottom left.
Scroll down until you see an option for HDMI passthrough. Turn it on or off to see if this helps.
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