Mumble
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Mumble is a free, open source, low latency, high quality voice chat application.
Mumble was the first VoIP application to establish true low latency voice communication over a decade ago. But low latency and gaming are not the only use cases it shines in.
We heard from users who record podcasts with our multi-channel audio recorder, players seeking realism with our positional audio in games, Eve Online players with huge communities of over 100 simultaneous voice participants (I bet they make good use of our extensive permission system 😄), the competitive Team Fortress 2 community making us their required voice communication platform, hobby radio transmission users, and a variety of workplaces adapting Mumble to fit their needs - be it on-head mobile devices or communicating across countries or into airplanes.
Administrators appreciate Mumble for being able to self host and have control over data security and privacy. Some make use of the extensive permission system for complex scenarios (for example separating two groups but leaders being able to talk to both). Some love to provide their users with additional functionality with scripts making use of server APIs, or host music bots and the like that connect to the server. Those that have an existing user database often make use of authenticators to allow authenticating with existing account login data.