I've had problems like this too. In my case, another program (TeamSpeak2) was using my audio. SWBF2 couldn't handle sharing the audio with TeamSpeak, and decided to crash to the desktop when ever I tried to play a level. Make sure you don't have any other audio or video programs running. Close any running programs and try the Beta again.
I fixed my problem by using two sound cards (a PCI card and the one embedded in the motherboard). SWBF2 uses the PCI card, while TeamSpeak's mic input goes through the motherboard's sound.
Though for some reason, SWBF2 randomly swaps some sound setting causing me to move TeamSpeak's, and the mic, to the other sound card then randomly back again.
(I may not agree with thoughts posted under this name)
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