HD Audio through HDMI? M3A78-CM

I have a M3A78-CM motherboard. It has what is call "HD audio" integrated in it.

Im a newbie by the way so bare with me.

The computer runs great motherboard and all. I finally got my DVI to HDMI chord in and plugged it in. I also bought audio adapter (s/pdif?) ( im using TV as my monitor) to plug in the back.

The problem is when I plugged my audio in the back of my computer (8-port audio) it was really low coming out. It can go through my TV fine but was really low in sound. So I decided to see what happens when I plug it in my front audio in. The sound works great.

I looked up the problem on asus and im not getting a clear picture as how to fix the problem. From what im thinking im understanding is that my audio can go through my HDMI with the type of audio card that is on the motherboard some how. Im using LINUX as my OS. Any suggestions?