Mumble

Mumble is a voice chat server built around low latency, positional audio, and an open client server protocol, so players hear sources attenuated by distance and direction rather than a flat mix. It provides channels, per-user permissions, text chat, and a plugin API for custom functionality, and clients exist for desktop, mobile, and the web.

Gamers use it for team coordination during matches, and it also works well for voice rooms around a shared game world.

Running the server on your own machine on your own infrastructure means audio traffic and player names stay off third-party infrastructure, there is no cost per channel or member, and the small server state is trivial to back up.