Hi major_tom. I'm afraid you can't get surround sound in Vista at the moment, unless you use a Creative Audigy or X-Fi series card.
This is a known issue with Vista and OpenAL (the lib we are using for sound in the game). Microsoft removed DirectSound3D in Vista, which is what OpenAL uses when it wants to open the sound hardware and a non Creative card is found. There's lot of info about this
here
So in conclusion, I am afraid if you want surround sound and don't have a Creative card, you would have to switch to Windows XP. This might change in the future though.