Just a guess here, but as soon as you mentioned HDMI out from the PC I immediately thought of HDCP being the issue. I ran into all sorts of issues trying to get Dolby Digital or DTS out of my HTPC over HDMI. I finally gave up and used the HDMI output for video only and ran an optical cable from my Creative Labs X-Fi sound card to my Denon receiver for the audio. This works great and I am using XBMC without any customization. Do you happen to have a digital output from your sound card that you could try?
