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