After connecting the TV via HDMI, restart the PC so it can properly detect the TV. I had problems with sound when using HDMI to hook my PC to the TV, i tried everything and nothing worked unless the PC is restarted after you plug the HDMI cable.
and this:
Select the GPU's Digital sound as the default audio device.
On windows 7: right-click on the speaker icon next to the clock, select "Playback devices" (sorry if not translating correctly, my windows is in portuguese), then select the graphics card digital HDMI output as the default sound device. It's name should be something like "Digital Audio (HDMI)".