sound cutting out on some channels

Sound and picture problems on multiple sets and multiple channels happen when the signals for the channels you are trying to watch are too weak or have 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 boxes 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 boxes/DVRs/digital adapter power cords for a minute or so.

If you can't find the problem or you'd rather have Comcast take care of it, call them at the phone number on your bill or 1-800-Comcast, or use one of the options on https://www.xfinity.com/support/contact-us/. Ask them to check the account setup and send a refresh signal to the boxes. 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 their Service Protection Plan (https://www.xfinity.com/support/articles/service-protection-plan, closed to customers that don't already have it). If the trouble is due to a faulty Comcast cable box/DVR/digital adapter or anything outside your home, you shouldn't be charged.

I don't understand how bad cable connections would cause problems on only a few channels and not on all of them.

I don't understand how bad cable connections would cause problems on only a few channels ...

Other causes are possible, but that's the most likely. RF signals behave differently from DC or low frequency AC, especially in coaxial cable.

As stated above, if you can't find the problem, the next step is contacting Comcast.

Here's what I found fixed the intermittant sound problem on some xfinity TV channels.   I had recently installed a Roku device on the other HDMI port of my TV.   Because of that, now I have to power on my TV with the Roku remote first.   Then switch to the Xfinity remote for selecting channels and adjusting volume.   That fixed it.