Strange, this exact same thing literally just happened on my TiVo tonight on all my OTA channels. (Cue LOST horns and/or Twilight Zone theme).
It was odd, the sound would get quiet and muffled, then get loud, then back to quiet. Maybe it's just a coincidence, but it sounded just like you described. Fortunately, it seems to be working fine now after restarting my TiVo and my a/v receiver...