Random muting

I have the polk sb 5000 and using the optical audio cord plugged into the tv it randomly muted itself.
I’ve tried having the tv speakers on mute, tried having it max volume, and everything else.
First time ever using the optical output.
I know the six works but this tv doesn’t have it and my ones that do I can’t mount in my living room so it’s not a simple switcharoo situation.

Answers