I'm sorry to hear that you're having this issue.

The most likely culprit would be the optical cable. I'd recommend swapping it out with a different cable to see if that solves the problem.

The other option, before you run out and get a cable is to call your cable/sat company and ask if they can reset, refresh or if there's a firmware update available for your box. Often times they can initialize a reset or an update while you're on the phone. This may fix the problem, and often, does.

(It's the simple stuff, sometimes!)