It is not the fault of Virgin's equipment if its customers expect state of the art sound quality when in fact even for some HD channels the sound is in stereo, which may account for customers experiencing variable sound levels.
It is understandably expensive for TV companies to provide content in higher quality formats and whilst digital sound may very well become an industry minimum standard, I suspect it will perhaps take ten or more years before it becomes the norm. After all HD has been around for quite a few years and the TV companies are slowly getting around to making this the format of their primary channels.
To summarise, if you are experiencing sound issues on your V6 box then change the audio settings to Dolby Digital to PCM and set your TV to Stereo for the vast majority of channels, otherwise you may experience issues with sound levels.
You can't learn anything from a mistake until you admit that you've made it.