Yet another one of my experiences trying to get something that works on Windows to work on Ubuntu.
I’ve been using TF2center.com quite a bit recently to get in some practice matches, but it’s not as smooth a process as I want it to be. It took me awhile to get the steam:// connect links working properly, but Mumble has been manual for the most part.
What I’ve managed to do is get Mumble to at least open, but the full URL is not yet fed. Launching Mumble via a URL will only open the application, not connect you to the server you wish to be connected to, or give you a username/password.