Newest drivers and all?
Are you watching movies, or are you talking about regular computer audio?
For Windows 7 (if you don't have that let me know and I'll look up the instructions):
Right click on the white speaker icon (bottom right corner).
Select "Playback Devices".
Click (or double click?) the "ATI HDMI Output".
If necessary, set as default or configure the options to get it working.
Hope that helps.