Well a lot of reading & messing about later & I think I have it nailed.
Over the weekend, I decided to do a fresh install of Vista. Since I know that I had 5.1 & now don't, something has intervened to change the audio stream.
Anyway, fresh install gave perfect sound. Add SP1, still perfect sound. Now here's the thing, MS issued an update to media centre called tv pack. Essentially it allows users to mix tv sources whereas Vista & previous versions would allow analogue or dvb but not both. This means in practice that I can hook up freeview and sky together and use HD as required. Tv pack also changed the audio plumbing & it's when I add this that the surround disappears.
However, I have had surround audio with tv pack previously so I still can't say for sure that tvp is the issue, but it is definitely somewhere between tvp & the soundcard. Tv capture cards do not process audio in any way, they merely pass it on to whatever audio device is in the pc. The soundcard splits the signal in to stereo & sends it out in what is effectively a contrived signal. Somewhere in that processing, there is a bug.
So I'm still stuffed, but I more or less know why I'm stuffed.
