This really depends heavily on what parts of #Discord you're trying to replace, as nothing currently has a 1:1 similarity with it. #Matrix will likely get you most of what you want but self-hosting it can be quite a sizable task. #Mumble is an excellent choice if your main concern is voice rooms, text chat is not so great. The ""best"" option would probably be to mix mumble with xmpp, as both do what they're designed to do very well and are also quite easy to maintain.