oops, sorry, I misread that you had fixed the problem.
HDMI is definitely the culpret, and no-sound issues are one of HDMI's many nettlesome issues.
You might try unplugging the power to the set top box for 30 seconds, plug it back in, and see if that fixes it.
But if it does, I can guarantee you that this will happen from time to time, and for the time being, that's the fix. HDMI holds great promise for ease of connectivity and use, but the bugs are hardly worked out, and keeping the HDMI connection happy can be a major pain.
Unless you really need the HDMI connection and are willing to put up with its occasional hiccups, you might consider using the 5 wire component video hook up. You will get the essentially the same full HD performance minus the HDMI hassles.
Hope that helps, let me know if there's anything else I can do, and thanks for using JustAnswer!