There is an issue with certificates for mumble, you wont be able to get on mumble unless you download the new EXE from http://blog.mumble.info/important-updat ... le-1-2-3a/ or use a new installer (currently not one available).
Just replace the downloaded EXE with the one in your C:\Program Files\Mumble folder. (mac users can still go to hell figure it out on your own )