Audio problems on HD DVR

Within the past few days my HD DVR has had audio problems. Some channels never have audio. Grey's Anatomy last night had perfect audio during the show but quit during the commercials. A Glen Beck show that we PVR'd and played earlier now doesn't play sound. What's changed in the past week? Or has my DVR developed a problem? It is very strange that shows have perfect audio but the commercials don't have audio.