I've been having a problem with dolby digital on HD channels for the last 6 months or so. This is especially when recording rather than live broadcasts. The solution I found was to go to the audio settings and change from Dolby Digital to PCM. You will then recieve sound on the HD channels. While I'm here, when will Virgin fix the recorded sound issue?????
Ultimately long story short Virgin sent out an update and it was detected a fault before it was rolled out further but they continued to roll the changes out. The U.S. also had this issue and sorted it in several days and rolled out a new version that fixed it. Her in Virgin land we still await that and they have even gone to the trouble of removing the fact it should do dolby sound in the specs on the Virgin website after they broke it. Not fix seems to be in sight. Ridiculous. There is a work around with another piece of hardware as mentioned in that thread which if you want true sound then you should go for as the difference is clear. PCM is virgins official 'fix' but it's a fob off really.