I'm running windows 7 64 bit build 7100 (fancy aero interface crap was turned off just using win98 style plain grey when it happened to me) with an Nvidia 8800GTX, Asus P5Q something or other motherboard, using sound built into motherboard.  Like I said I'm pretty sure that only happens to me when I have the editor running too... but I could be wrong and maybe it's hardware or driver related.  The atari-blah_mat is the materials package... I didn't quite get/understand in the beginning that the materials would be packaged/compiled into the .ut3 map file... plus that LOC file thing... I don't totally get everything about map making but hey you're the first person to have a legitimate problem with it that I've talked to.  Hopefully it will be done doing that 3 minute loading shit after the first time it loads.