Re:Benefit of using sound card instead of HDMI port on graphics card when using receiver?
As far as I know, if you're sending an LPCM signal, meaning a non-DTS HD or TrueHD audio signal, both cards should be sending the same information. However, most Blu-ray movies use DTS-HD and TrueHD, and without the Auzentech card (or equivalent), most programs can't decode those signals.
The Auzentech card does a hell of a lot of processing work and can even re-encode the signal sent to your receiver. Since you've got the card, I'd just use it and not worry about the ATI HDMI thing.