Richard's optical speakers are suddenly malfunctioning with only one speaker working. Leo says if the wires are connected, then it could be a bad speaker wire. They can fail. The speaker can also fail. Do the easy thing first ... replace the wires. Also, try using a different device like a turntable. That will eliminate the receiver. Also, change the connections from left to right. If the other speaker goes out and the current speaker has sound again, then the AV receiver may be going out. The AV receiver may also have a testing function called "Audacy," which would enable him to plug in a mic and test the sound configuration. It should come with the Mic. If so, then run it. It could reconfigure and eliminate if the speaker is just bad.