I'm on Comcast in VA, so I thought I'd test a few of the music channels. I'm not hearing any dropouts.
My set up: cable from wall to splitter; splitter to +15dB signal booster, then straight into the PCI-e InfiniTV 4's dongle/mini-coax cable (other split to cable modem). I'm using the HTPC's onboard coax digital out (typical Realtek) to a digital audio switch (4 In, 1 Out), and finally Optical Out from the switch to an old Kenwood VR-6060 AVR. That's a lot of stuff in the chain, but no problems.
Could it be your AVR, if you have one? As a test, if possible, you could hook up a pair of self-powered PC speakers and make that device the HTPC's default audio device. Or use the TV's speakers.
Maybe it's Comcast. In VA, Comcast bought Adelphia, which was a Scientific-Atlanta DVR area. I know Comcast butchered the software/firmware in the Sci-Atl DVRs to match their majority Motorola areas. That's merely to provide a possible point of reference. Maybe it doesn't matter in the end.