did you ever find a solution? its hard to follow that old thread but what your exact issue was still seems foggy as Im looking for a similiarly related topic and came across this thread. if you were having issues with the audio/visual thats all on the onboad/soundcard, I have a similar setup so your whole issues sounds like motherboard to me. (either mising mb/soundcard/DSP drivers or outright broken) My problem is I cant get native 5.1 output thru my 5750, it'll do it if I use the onboard hdmi but not thru the card. Eg. AC3 and any 2ch dolby works but if its pure 6ch it cant.
Also I seen you mentioned asus, dont look for the audio for your 5750 hdmi to show up at all if your looking for it in the VIA HD audio deck because it dont.
You have to pick the output thru window's mixer << and that relies on proper mb drivers installed but not the software. o.O
I seen you just did a fresh install likely that was caused by drivers both on the 5750 driver pkg and the MB/Codec chipset. One thing I learned is ATI and codec package drivers you always have to use a 'cleaner' after removing, then rebooting, then clean, then reboot again, then install mb/codec first, reboot, then do GPU drivers, reboot last time... done....