For musical purposes, mic is a noun, whereas mike is a verb. For example, a singer puts a mic in front of his mouth and sings into it, whereas a roadie's job might be to mike the drumset (stick a bunch of mic's in front of the various drums and cymbals).

When referring to more than one microphone, I'm pretty sure you use an apostrophe, so as to prevent people from reading it as "micks."
See: P's and Q's