Image from http://exosquad.net/mumble/
What you'll need to do is open Mumble and then go to the 'Configure' option. Once inside, select 'Shortcuts' on the left-hand menu. Then you'll need to click the 'Add' button at the bottom. This will give you a new shortcut in the 'Function' column with a drop-down menu. Click the down arrow to select 'Deafen Self.' The 'Data' column will automatically default to 'Toggle'; this is what you want. Now, click on the 'Shortcut' portion and enter the keyboard shortcut you want to use. I personally use the tilde key '~' because it's not really used in-game and yet you have easy access to it. The 'Suppress' options makes the key activate despite game commands, so leave this unchecked; sometimes this causes errors in the game. Click 'Apply' and you're done!