Mumble
Mumble is a audio group chat / VoIP application. It is designed for gaming communities but is also useful generally for calls, meetings, hangouts, etc. It uses a client-server architecture with encrypted connections. There are client applications available for Linux, Mac, Windows, iOS, and Android.
Sudo Room Mumble Server
Sudo Room has a Mumble server. You can connect to it at address room.sudoroom.org and the default port (64738). The server runs uMurmur and is hosted on the saros machine. This was set up by cel.
Channels
A Mumble server can have multiple channels. If you are on the server for a specific call, you may want to make a new channel separate from the main (root) channel. In the desktop Mumble client, you can create a temporary channel by choosing "Add…" from the "Channel" menu, then type a name and and optional description, check the "Temporary" box and press OK. Other people can join the channel by double-clicking it or dragging their name onto it. When everyone leaves the temporary channel, it disappears.
Audio settings
When you first start Mumble, you should use the Audio Wizard to configure your audio. To reduce feedback (echo), every participant should either use Push-to-talk mode, use a headset/headphones, or enable Echo cancellation in the settings. If you have a scheduled call and have not used Mumble before, it may be helpful to connect early to make sure your audio works.