Honestly I would guess you might want to lose the OS. You are running Server 2008 as your home operating system. Generally hardware vendors dont develop drivers for server class OS's using user based hardware. You might have more luck running XP, Vista or Windows 7.
It is extremely relevant of the hardware you use for your server if the OS does not support the hardware. You can't just go around slapping vista drivers on a server 2008 OS. It does not work that way. I do know the definition of a server. Much better than yourself or you wouldn't be using server 2008 on a normal desktop PC and expect all of the drivers to function properly.
I hate you kiddies that think you are cool by building what you think is a server in your home but not even close to understanding what to do, how it works, and what you need. Install vista, xp, or windows 7 and I will gaurentee your sound will work properly.
Not to mention the fact that the problem you are having is sending audio over an HDMI cable? Buddy servers don't hook up to HD TV's. Your looking for more of a media center.