No Sound Switching Channels

HD installed week ago

Probs experienced to-date with getting sound when switching channels (SD to SD & HD to SD say)

To restore sound have to flick in & out again from channels ('hit & miss' approach) - background sound always present though when going to any function button - fault with thomson box as why would amp still give the background noise all the time even though channel selection has no sound

Sound settings are set as stereo/full volume/dolby digital (with using hd box leave as DD ALL the time?)

Never experienced any probs with the former sky+ box with sound

Notice as well say watching sky sports HD the sky channel info banner does not show DD whilst other HD channels display DD most of the times - why don't sky display correct sound output with their info banners

Sound levels (0-60) thro the amp poor compared with previous levels used to hear with sky+ box. Different channels give clear sound output @ low levels woth others it is non-existant etc