Why does the sound on some cable channels go in and out?

This depends on how you receive these channels that this happens on.

If you receive them via the tuner in the TV: 1. It may be the channel from the broadcasters, or cable provider. 2. It may be the tv tuner going out/drifting.

If you receive them via a Settop Box STB (cable converter): 1. If you are using HDMI it is likely that the communication between the STB and TV via HDMI is faulty, or version mismatched 1.2 to 1.3a (some TV manufacturers have a firmware/software patch that you can use to correct this). 2. If you have a STB that has Dolby digital setting, make sure it is turned off.