Mumble is a free, open-source, channel-based, voice and text chat software.
1. Download and install the Mumble client from mumble.info or here.
2. Add a new server, enter the address "mumble.nexunity.org", any label and your username.
3. Connect to the added server. Alternatively use this link to connect.
4. For more information see this video and the Mumbleguide (English and German).