I have a Sound Blaster X-Fi Xtreme audio card with Creative 5.1 speakers.

My game would initially start up with no sound. I could immediately renable the sound if I went into the settings and switched the source over to something other than whatever it was currently set to (3D Sound to Safe Mode, EAX to EAX+, Safe Mode to 3D, etc).

I go to fire up my game today however and there is NO SOUND at all. Nothing. The prior trick no longer works. I try deleting my INI file, I try reinstalling the entire game. Nothing.

Everything is hooked up fine. Everything works great. This problem only exists within UT2K4, and has apparently always existed to some extent or another. I'm tempted to try installing from CD instead, but I don't want to dig out all 7 discs when I have my GOG installer on my desktop...

Hmm, sounds like a sound card issue. Have you tried switching to your onboard sound rather than via the X-Fi soundcard? If you can do this and it works then at least we know it is definitely something to do with the soundcard.