Go into your control panel and locate the Sounds and audio devices section.
Change the default device to the ATI HDMI Audio device (not sure of the exact name, just look and you should locate it).
You should now have sound over HDMI
Thank you. I was so happy when I figured that out. I can now actually play 2 movies at the same time using one pc. Been wishing this for a long time. I didn't know ATI 5800 series actually have a built-in sound card chip.