What GPU (video card) are you using and are its drivers up to date? Also are the HDMI audio drivers installed? Last (not to insult) make sure the volume is actually turned up on the HDTV (you wouldn't believe how many times this is the problem....been there, done that).
AMD/ATI cards starting with the Radeon 4xxx series has on-board audio. Therefore all that is necessary is to install the audio and video drivers for the Radeon card and you are good to go. I think sometimes the audio driver is not automatically installed from the setup disc and in those case you have to manual install it yourself. You should also uninstall drivers for any sound card or integrated motherboard audio.
nVidia cards do not have integrated audio. You must connect a "pass-thru" cable from the motherboard or sound card to the nvida video card so that the audio signal can be passed through the HDMI cable.