I've got a core with a GT430, had it for a few years now and never managed to get the audio over HDMI working. I was using an optical input to my amp to get around this problem but it was frustrating. Nowdays I use a Zotac MAG as my core because it does just as well as the monster PC I was using, and uses a fraction of the power.
I know it's no help for you, I had many threads on here trying to get this working over a year ago, but if you do manage to get everything working then top marks to you, and please post something in the wiki so that others can follow it later if necessary.
Cheers.
Matt.