ATI HDMI audio via HD 3870 to HDMI TV HELP no sound!

This will be written assuming you are using Windows 7.

Go to Start > Control Panel > Sound.

On the playback tab, there should be multiple options.

Right click on the one that coincides with your HDMI / TV and choose Test to be sure that it is working. Also choose "Set as Default" to always use it as the default audio source.