Mumble

Mumble is a voice chat client built around keeping the delay between speaking and being heard short enough for coordination during games or live work. It connects to a server program that a community runs for itself rather than to a hosted service, so a group provides its own infrastructure and controls who may join.

A server is organised as a tree of channels. Moving into a channel means hearing and being heard by the people in it, and access is governed per channel by access control lists and groups. Audio is carried with the Opus codec, using push-to-talk or voice activation, with adjustable noise suppression and echo cancellation. Positional audio places other players' voices according to where they are in supported games, and an overlay can draw who is currently speaking on top of a full-screen game.

Traffic between client and server is encrypted, and users are identified by a certificate held in the client rather than by a password. Text messages can be sent alongside voice, but Mumble is not a general messenger: there is no offline history to catch up on after reconnecting.