|HDMI is nothing more that DVI & audio sharing the same cable. You're not going to get sound thru the HDMI port until you connect the sound to the video card. HDMI is generally used with home theater systems. It's main advantage is to cut down on the cabling.|
What are you connecting the HDMI cable to...a monitor? with cheesey monitor speakers? If that's the case, don't bother trying to run the audio thru the HDMI. Just plug your speakers directly into the audio jacks on the motherboard.
Have you installed the latest nvidia drivers for your card?