Are you running a CacheCard? If you are then you might like to check the firmware revision of the card. More information can be found
here.
It is also handy to know if changing the channel solved the sound issue. For instance, if there is no sound does changing the channel reset it? Have you ever had the sound disappear while watching a show... does it sometimes disappear partway into a show, or always at the beginning? These questions will also help determine if it is a software versus hardware issue.