I have played around a little with Mumble these last few days after seeing your post (using an Android client and accessing public servers, so nothing Discourse related) and it’s pretty neat.
The plugin in OP in only a “viewer”. Users require a Mumble client to access the server and use it. There is then the “authentification” problem: You would want your forum users to show with their name in Mumble and know it’s indeed them. One guy was already searching for a solution to this 6 years ago here on meta, it seems: $100 Bounty: Create script or plugin that links Mumble to Discourse's user db
When browsing around, I’ve encountered a forum, which asks its user to join their Mumble server and then send a PM to a moderator on the forum, and they validate the users manually. Seems to work, but it’s MANUAL.
An integration with WebRTC so that you don’t need any client and you have directly everything you need on Discourse would indeed be amazing. If you could use it publicly (for public discussions) and privately (for one-on-one or multi-users private discussions), it would be incredible.
I’ve seen discussions on meta about some video-conference integrations lately. Is there something which would already do this ? (hopefully with the video portion as optional)
A guy was interested to do something with Mumble 2 years ago, but it didn’t get much love (he was mainly interested by one-on-one private discussions):