Going out on a long shot, audio drivers or codec interference issues. Update the audio drivers and add UT3 to any exception list if you do have a codec pack installed like ffshow.
Another suggesting is trying is a clean OS install or different OS. Partition one of your drives and load up a second copy of your chosen OS or Win 7 RC would be a good choice too. I'm having ongoing Fallout 3 issues and the early CTDs were solved by going to windows 7 RC to play the game. There is one reproducible issue Win 7 hasn't fixed and I've been able to escalate the issue past tech support to the developers but am still waiting for an answer and its been 2 weeks since tech support handed it off.
If you haven't spent any time on this forum, they have more answers than this site would: http://utforums.epicgames.com/forumdisplay.php?f=342
And if that can't help, try emailing Epic Tech Support.