Long Shot, you never know, you might get lucky:
What if a button that engages MUTE is stuck, on the remote or the unit.
You could take the batteries out of the remote to see if you get lucky. If a MUTE button exists on the unit, try taking the cover off, see what moves or doesn’t move.
I got lucky a few times, by taking remote controls apart, hosing the board/contacts, all moving parts with alcohol, cleaning all the traces/contacts with a small artist’s brush, blow dry with my compressor, working again.
Sometimes, the rubber buttons don't move right, gummed up with peanut butter or jelly.