lol... a soundcard is not going to help with that.
I forgot to tell you one thing about sound on a Mac. All sound goes through the CPU on a Mac, EVEN if you add a soundcard. This is the reason the Mac is great for sound applications. A soundcard is not going to help the problem you describe. This is different from a PC where a soundcard offloads the sound from the CPU.
Now saying that I can't understand why you would have sound problems like the ones you are describing on a Mac Pro, unless WOW is single-threaded, even the sound, but I don't think that is true. The sound should be going to another core than the game. Unless it's different than UT2004 which separates the sound to another core. (I don't play WOW) I've read alot of performance issues with WOW, but I thought they were limited to graphics.
There are lots of soundcards for the Mac, the problem is they are mostly PCI (or PCI-X), so they work great in the PowerMac G5, but not the Mac Pro. Alot of Audio experts use the Mac for professional Audio, even the movies. Ever heard of Star Wars. All the latest Star War movies sound was recorded and mixed on a Mac (I believe the recording part was done on a Powerbook). Alot of bands record audio and mix it on the Mac. So don't knock the Macs ability for sound.