Press the LANG button on the DTA remote. Do this on a channel that does not have sound. Check the rest to make sure they do also, you may have to press the LANG button on each channel that has no sound. Sometimes the signal coming in thinks that it is supposed to be another language or in SAP mode. I had a channel lose sound in the middle of a program and pressing LANG worked.
I have Digital Transport Adapter Model DC50Xu installed and was working for weeks, yesterday (1/10/2011) the voice portion of dozens of channels has disappeared. I went online (comcast.com/digitalnow) to reload but this didn't work. I tried to call 1-877-634-4434 but the line answered but no one was on the other end. Any suggestions?