basiles :
Are you sure you followed my instructions? Go to Control panel, sounds, manage audio devices. It will open a dialog box which lists your audio devices. you need to click on the device you want the sound to come from and the "set default" button should not be greyed out if the selected device is currently NOT your default device. If you have done all this then I'm afraid I cant help as the HDMI connection should carry both the sound and video.
Ok just like you I was able to change my sound out put from the on board card to the ATI Radeon 4350. But then for no reason it just dispeeared and now i cant select the HDMI out put for sound. THis means all I can use is my pc speacker and not my HD TV. I have an HP with windows.