How to enable your microphone if it's disabled in MATE sound preferences

I'm posting this so it shows up in search results for future me and others like me. The solution might be dead obvious -- but not if you're in a hurry!

Sometimes when you plug in a microphone, MATE sound preferences will show it as disabled (see attachment).

You need to reboot before PulseAudio will let you use it.

(If anyone knows a way to do this without a reboot, I'd love to know).