If I've muted myself in the first session, so that my USB-headset is connected to the computer and is in the use, and leave the session without unmuting myself and then removing (Sometime my USB-headset likes to do it own way) my USB-headset from the USB-port between the two sessions: Mumble will freeze after trying to connect to any server. The only solution I know for this is to close the Server List, go to the Options and change the Audio Input to something else.
It will also freeze if I close the server list and click on the unmute-button.
Pretty annoying, especially when I didn't find the solution. I had to delete settings/uninstall Mumble to get it work again.
Asus Xonar DX soundcard is in use, along with Creative HS-950 USB-headset.