cryo and I have had trouble getting our PTT key to work in Mumble lately. It will work fine one day, and then not work the next. I've googled this problem, and see that others have it, but I haven't found a solution.
Always On, and Voice Activation do not work either.
I have it as a key, and tried all of those things. Restarted, reinstalled, etc. Eventually it started working again (after I quit for a while), and not through anything that I did, as far as I can tell. This happened a few weeks back too, with the same resolution.