Need a copy of mumble ?
Grab it from http://mumble.sourceforge.net/
Works with Windows, Mac OS X, and Linux (with QT).
Mumble connection info
Servername: Something that you'll remember is us! MCPublic or RedditMC are good ones.
Username: Your Minecraft name
TURN ON PUSH TO TALK
When you first get mumble it will talk whenever your mic has activity. This is extremely annoying and you will be kicked if you keep it up.
To change it, go into Configure -> Settings -> Audio Input -> Change Voice activity to Push to talk
Next, you have to set a shortcut for your push to talk key, to do so, Configure -> Settings -> Shortcuts -> Add -> Function: Push to talk -> Shortcut: Key of your choosing.