With our awesome Minecraft server host, Beastnode, they provide both a free web server, and a free Mumble voice chat server. Pretty sweet for the price. I figured it was worthwhile to setup since we are trying to build a large player community.
You can connect to the Mumble server @
We are still working out some of the kinks with it, as we are used to Teamspeak, but it was free with our server, and it has 64 slots, which allows for a full server plus some :)