Mumla is a client for the voice chat system Mumble. Connect to one of the many public Mumble-servers run by the community, or host one yourself (see https://www.mumble.info for more info). Mumla is an updated and maintained version (a fork) of the app Plumble.
I downloaded and tested it. Mumla seems to not want anything from me, no attention, no data and no currency. You can join public servers and even host your own.