Even if not everyone is talking, that's not the issue. If not everyone can hear listen to calls, even ingame communication is better.
Make it possible so lobby leader can have the option to set his lobby mandatory with mumble or not. Once the lobby is up, make sure on the front page theres mumble icon indication at that lobby , so when ppl hover either the icon or the lobby a text will appear and says mumble is necessary to play.
if ppl doenst have mumble and still join the lobby make it so ur mumble server recognize that there are ppl missing and report the user. or make it so players can report.
If the user has too many reports then he isnt allowed to play on a mumble lobby for x days/week/month etc
Just thought it would be a nice option so that you don't get lobbies with 1 or 2 people from the team in the mumble