How can I output sound to my receiver through Radeon 6850's HDMI?

Hi folks, I'm somewhat computer savvy but am having a real difficult time figuring this out. Some info on my specs:

1. ASUS M3N72-D AM2+/AM2 NVIDIA nForce 750a SLI HDMI ATX AMD Motherboard
2. Radeon 6850 Video Card (Upgraded from a Nvideo 9800 GTX+) - It has a displayport, hdmi, two dvis
3. Windows 7 64 bit
4. Latest RealTek Drivers
5. Denon AVR2310CI 7.1-Channel Multi-Zone Home Theater Receiver
6. A 5.1 setup (have not bought the rear speakers yet)

I currently have my sound being outputted from my computer to my receiver using the onboard analog mobos 3.5mm connection that splits into a left and right composite cables. However, it only registers as a 2.0 system and my center, side speakers, and subwoofer cannot be tested in either RealTek control panel or windows playback devices.

So I'm trying to output sound via the Radeon 6850's HDMI port. The computer detects the receiver and does display an image on the display unit going from the receiver. However, the sound does not work. In addition, Realtek control panel does not show AMD audio like the windows audio playback devices does.

Any ideas on how I can get this to work?

Thanks.