Hook up the S/PDIF cable to the sound card if you have one.
Disable onboard sound.
Go into the audio properties
(which should show up as a red speaker icon on the start bar).
From there enable the audio via HDMI, and then go into properties.
This is the easiest pic I could find which shows all the information in one shot; http://i30.tinypic.com/iz9pvl.jpg