You're probably right, but we have a customer that is insisting that the sound card be Windows 2003 Server compatible, and I know many out there have problems with this OS.
I've run Windows 2003 Server before on a rig. Drivers generally run fine if they are designed for the 32-bit version of Windows XP. You aren't going to get very far with an attitude like that anyway. Try and download the ATI Catalyst driver for Windows 2003 Server... Well you can't!! You have to use the Windows XP Professional (32-bit) driver. That can be a bit flakey as I recall. Windows 2003 Server is designed as a server OS with minimal multimedia support by default. I mean you have to turn on DirectX support again to get accelerated sound and 3D graphics support. So don't expect to find sound or graphics drivers out there specifically for this OS. Your customer is an idiot.
If he really needs the server features in Windows 2003 Server (DHCP, DNS, Domain, servers etc.) then he will have to accept some compromises. Normally you would only need a basic 2D VGA interface to a server machine right?
Wouldn't Windows XP x64 be better if you customer needs multimedia support? It shares the same code base but won't have the extra server features...