There are even services that give you free temporary servers. I don't know why anyone would use that instead of just finding any server and use a free room if you just want to talk with friends, but well, it exists.

You go to a service like that, press a button, it generates you some random port number and password, then you connect to that server with mumble and become an admin of it. The server is temporary and gets automatically destroyed after some fixed period (usually something like 24 hours).

Also what I tried to do with my friends is run Yggdrasil and connect directly via IPv6 (so I run Yggdrasil and launch a mumble server, and all my friends enter my IPv6 address) and that works too, so no need to have public IPs or domain names to use Mumble anymore.

Yggdrasil is such a cool thing, loving it.