If you ever run into any visual or audio problems, or any other questionable problem with your Comcast cable, the easiest and first thing you should try is to simply reboot your cable box. Often times this alone will clear things up. There are two different ways to do it, one is a "soft" reboot, which won't clear it to factory defaults, and then the "hard" reboot, which does bring it back to exactly the way it was when sent from the factory, including removing any customizations / preferences you may have made. Always try a soft reboot first. Here's how to do both:
Just unplug the power from the wall socket for about a minute (60 seconds) or longer, then plug it back in. Simple.
Do the exact same thing as a soft reboot, but before plugging it back into the wall, press and hold the "Select" and "Power" buttons on the front of the cable box.
Note: This is for the Comcast HDTV Cable Box (Model DCH3200).