Well that's spooky...
Check this thread:
http://www.subsim.com/radioroom/showthread.php?t=156102
Exactly the same problem as you and guess what? Both using DBSM.....
Disable DBSM and test the game without it. If it works then I suggest using Thompsen's Soundpack, it too has Das Boot soundbites and I've been using it with GWX no problemo.