I'm going through your documentation now
So lemme repeat this, you already had a soundcard but aren't getting any audio output...and you think your thinking , you should buy a soundcard that is actually support with the x4/x8 slots...right?
Your slots available are x4 and x8's. All soundcards (that are x1) can run off an x4 or an x8 slot. The only catch here is that the x1 card will only need 1 (electrical) lane to run. So that eliminates any doubts on how they work.
About the drivers, If i'm not mistaken, you can update them in Windows serrver 2008. What is your maker for the card? I'll advise a new soundcard , once we figure out what's been with your old one
Found more info: http://social.technet.microsoft.com/Forums/en-US/winservergen/thread/1fa79b9e-94b9-4996-8400-6a22c77f19c2/
if these don't work, then i'll move about for a simple x1 audio card for you