Well after enjoying a week or two of trouble free htpc' ing (since ditching the 8400gs gpu and going with an Asus 4350) I hit trouble again. Pc failed to start, got continual beeping from it, ordered a new stick of ram, problem fixed! Except not quite, as then there was no longer any audio from the hdmi out, despite all settings indicating otherwise. I went back to using the riser card for a couple of days just so that the unit was functional, and settling down for what I excpected to be a long and frustrating afternoon of reinstalling drivers and mucking around with settings (yes there is actually quite a lot on forums etc regarding the 4350 and no audio) I was to be very surprised by the fix...
Having plugged back in the 4350 to set about fixing, on switch on it worked fine with audio etc. I assumed the pc had re recognised the card and that all was now well (until the next time anyway!) so switched off to screw the card down, but noticed that the screw holes didn't line up too well -I had to prise the card into place, but with a little determination it was soon all screwed down - a bit tight getting the hdmi connector on though as it rubs on side of pc frame. Restart, guess what? Yep no audio. (Also, interestingly, no bios screen - just straight to windows). I had of course unplugged this card and tried the riser card when initially checking that the original non start up fault was indeed the memory, it must have been this tampering with the card that set me up for the no audio problem.
Anyway, there it is - if you lose audio with an ati type card - and the bios screen stops showing - it coud just be that the card and hdmi connector are fouling on the scaleo case!