I have seen this problem twice on two completely different setups and was able to fix it both times. The first time I figured it out was actually forty minutes before the scheduled appointment with a Comcast technician. I contacted their tech support the previous day and they seemed to think that the only option was to send a guy and probably replace the device. Here is what I did instead.
It occurred to me that maximum sound is the sound to which no signal modifications has been applied. Wouldn't it make sense to bring the volume ALL THE WAY UP to reset sound decoding on a cable box? Whether or not this theory is valid, it works. I really wish Comcast told me about this trick. On my DC60x I simply raised the volume using the cable box remote to 60 (the maximum value). I suppose you could even mute your TV while doing it, so it's not too loud. Like I said, this worked on two different setups. My next door neighbors had the same exact thing happen to their cable box and I was able to tell them what to do over the phone (I did not bother going outside). This tells me that this is a common problem and that my solution will work for many people with the same issue. Of course, your experience may vary. Maybe you will need to have your cable box replaced. I did not, so far.
