Mine does that frequently. The cable box has turned mute on by mistake and I'm assuming your remote is firing the mute command at the TV or stereo receiver. You'll either need to make sure the cable box remote is set to control the cable box before you hit mute.
Or, I believe I once found a convoluted way around that by going to the Comcast main menu, selecting "setup", then "audio", then toggling the "optimal stereo" option.
But I would recommend programming your remote to make the mute button link to the cable box.