Mumble is a voice chat application for groups. While it can be used for any kind of activity, it is primarily intended for gaming. It can be compared to programs like Ventrilo or TeamSpeak. People tend to simplify things, so when they talk about Mumble they either talk about "Mumble" the client application or about "Mumble & Murmur" the whole voice chat application suite.
You can download Mumble here:
http://mumble.sourceforge.net/
Connect info:
address: mumble.planetminetest.com
port: 64743
Note that this can be used for all the servers in the MT community. If you would like me to create a channel on the server for your server just send a pm or post in this thread. If the server is used by the community i will upgrade it to more slots. If not it will remain 10 slots.