Well...sometimes there is sound!
Hi
I've been having the same problems as others. I tried all of the various suggestions here - thanks for posting helpful suggestions - but nothing fixed the problem. Still no sound. However, at Christmas I unplugged everything from my Mac Pro and locked it away before going on holiday. When I returned 10 days later I got my Mac Pro set up again, plugged all the cables back in again and booted it up into OS X. Everything worked fine including my Soundsticks (they always work in OS X anyway) Then I rebooted into Windows by holding down the Alt key and when I logged in to Windows I heard the XP login tune playing through my Soundsticks, which were plugged into the rear Audio Jack socket - the Line Out!!
So, without installing anything extra, the audio suddenly started working from Windows on my Mac Pro. So, I am convinced that the problem has nothing to do with drivers or service pack version of XP, etc. I was able to shut the Mac down completely, boot between OSes, etc without losing the sound in Windows. Game playing was back and sounding better than ever on Windows!

Further to this, the sound worked beautifully on Windows for about 8 days until one day I needed to access the rear of my MacPro to plug something in to a USB socket. I gently eased the Mac forward slightly to allow me to get my hand in round the back of the Mac and as I did that, a little bubble message popped up in Windows. It said "An audio Jack has been unplugged" . "No it hasn't" I thought to myself but I checked to see if the jack had been disturbed by me moving the Mac slightly, but it was in fully. The sound had gone again

I booted into OS X and the sound worked perfectly. I booted back into Windows again, no external sound, just back to the horrible tinny sound of the internal speaker. So I shut down, unplugged everything, left it for a while to simulate the conditions under which I got it to work before, plugged it all back in again but no joy. I'm back to having no sound again in Windows. Damn!
What I can tell you though, is that it IS possible to get the sound working without messing about with drivers or enabling anything in Windows Control Panels. If I get it working again I will report back. In the meantime, if anyone else has managed to get their sound working in XP, please please let me know how
Thanks
Col