Probably because no one has an answer for you.
Doing a google search I found this:
"dual 1.33GHz G4 Xserve that is fitted with a M-Audio Audiophile 192 PCI card. This works well but is expensive new (I got mine used). The only complaint (as a general purpose sound card) you have to use the M-Audio app to change the volume - the normal volume controls don't work."