I am using a DCT3510-M and I have been having a number of audio and video issues over the last couple of weeks/months. The main thing that I have noticed is that the sound will always seem to go away during a fast forward on a recording. It will also fail when you press the "skip ahead 30 seconds" button as soon as the recording playback starts.
Just right now I was fast forwarding and not only did the audio quit, but then the whole machine rebooted itself.
When the sound stops working after fast forwarding it will not only affect audio on the recordings, but it will stop the sound from working on the live channels too. A reboot will always fix the problem.
This seems to have started happening since you changed the network to require a digital box. I have a digital box but I know you made the change in my area because none of the tv's at my gym work anymore. (I'm still mad about that too, but that is a different issue)
Please fix this issue. I am seriously starting to wonder why I pay so much for cable and this box and the "new" HD software have been nothing but problems since you first introduced it