I have the same problem. Usually if I hit the language button on the comcast remote, it will bring back the sound. Other times, if I hit the mute button on and off....even though you didn't mute it. Hope it works.
On one of the channels with no sound press the LANG button on the DTA(small box) remote. Sometimes the signals make the unit think it is in SAP mode.
I had it happen to me on one channel with one of those boxes in the middle of a program. Some people say that the LANG button gets pushed by accident but when it happened to me the remote was nowhere near my hand so it had to be a signal issue going into the box.
i have one of those little digital comcast boxes on one of my tvs and the other day on 2 channels i get no sound. now this has expanded to 5 channels with no sound. my other tv is ok. would this be the box going bad or tv, i am thinking more the box.
NBC and other select channels up the chain do not have sound whereas the other channels do. It is very odd and this is now the second TV that does it, but the first one that has one of the new digital boxes (the smalld black box).