Good news, I just got mine to work! Turns out that my PC wasn't recognizing the speakers on the screen. Go look under Sound > Playback Devices. Make sure there is a "VX238" there. If it's missing, that's probably why it wasn't working.
I remember mine didn't have a "VX238" at first too. I don't know why, but for some reason last night when I reset my screen and when ATI Catalyst asked if I wanted to configure the HDMI Audio device that was detected (which I've been selecting "Ok" the whole time and nothing happened)
, I clicked "Ok" and this time the speakers suddenly just started working. Also, a "VX238" appeared under my Playback Devices when I checked, which was why it started working.
Maybe you could try fiddling around with it and see if it would randomly install/recognize the HDMI Audio Device. Maybe plug in and out the HDMI cable a few times. Or maybe reinstall the whole ATI Catalyst Control Center if you're using ATI. Theoretically, that might help since it reinstalls the "HDMI Audio Driver". If you're using nVidea you could try that too. Though I'm unsure if it's the same.
Let me know how it goes