I've looked at the Mumble Sourceforge forum and I didn't find anything, so I thought I'd ask here.
I've been playing Left 4 Dead 2 and Killing Floor with some friends but both games crash when I have Mumble running. If it's running when I launch the game then they both crash, if I alt+tab out of the game, launch Mumble and then alt+tab back in it crashes straight away. Doesn't matter if it's Fullscreen or Windowed. I've also gone into Control Panel -> Sound -> Communications -> Do Nothing on the suggestion of a post at SourceForge, but that didn't help either.