Have you tried searching? I seem to recall other people discussing using GPIO on a Rpi to control Mumble, your question might already be answered by people who know more about it than me.
It's possible to control some aspects of Mumble via an RPC interface that's not terribly well documented. I know some people have had some success running Mumble headless with a virtual framebuffer X server - you'll have to configure it with a GUI at first though I think. Again, this should be a solved question already - if you can't find anything on these forums, check the SourceForge tracker for discussions as well.
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