This works in Richmond, VA - Comcast,
1) Turn on TV & box, verify you are on the correct input.
2) Turn off cable box only.
3) Point Comcast remote @ box and tap "Menu" (make sure remote is in cable mode)
4) Setup menu should appear on your screen, navigate down to audio adjustment and try a different setting (I'm not looking at one now so I don't remember what the choices are). You may need to go into a sub menu entitled "Additional HDMI settings". You can't brick the thing by changing any settings in there so feel free to experiment,
5) Turn off box, then turn back on