Pixelation and loss of audio happen when the signal for the channel you are trying to watch is too weak or has too much noise. This might be because of a bad cable box or a problem at Comcast (check https://www.xfinity.com/support/status/, 1-800-Comcast voice response, or the "My Account" app) but is most likely due to a poor connection between the box and Comcast's network, usually in or near your home.
Troubleshoot by checking all connectors for corrosion and tightness, and by looking for damaged coax cable. Running the cable through a surge protector, a defective splitter, or too many splitters can cause signal problems as well. If there is an amplifier in the line make sure it's getting power. You might also try unplugging the cable box/DVR/digital adapter power cord for a minute or so.
If you can't find the problem, call them at the phone number on your bill or 1-800-Comcast, or chat with them using the TV Support option at https://support.xfinity.com/chat. Ask them to check the account setup and send a refresh signal to the box. If they can't fix the problem remotely, insist they send a tech out to identify the cause and correct it.
If the tech finds bad coax, splitters, amplifiers, or connections in your home (even if Comcast originally supplied them) you'll probably have to pay for the visit unless you have or sign up for their Service Protection Plan (https://www.xfinity.com/support/articles/service-protection-plan, about $6/mo) and keep it for at least 60 days. If the trouble is due to a faulty Comcast cable box/DVR/digital adapter or anything outside your home, you shouldn't be charged.
For additional information please see Pixelation, video and audio dropouts, One Moment Please S0a00 messages.