help me with a no sound issue

Just built myself a HTPC and am not sure what is going on with the sound or lack thereof..

It has a ASUS M3N78-EMH HDMI motherboard running Vista Ultimate and all drivers etc are installed. I hooked up the HDMI from the PC to the HDMI of the TV and can get video but no sound. So I grabbed my old 3.5mm to RCA cable and plugged it into the Lineout port and into the AMP but still no sound. I have been testing it on windows sounds so its not a codec issue.

The motherboard has the following 6 sound ports on the back:
Rear Speaker out
Centre/Subwoofer
Line In
Line Out
Microphone
Side Speaker out

The motherboard states its got High Def audio so how do I hook these up to my Dolby Digital II Amp? The amp has optical but the motherboard doesn't.
I also hooked up the front 3.5mm ports on the HTPC case as High Def Audio..

Any help would be appreciated..

Paul