It's the very odd nickname for a piece of electronic equipment.
For uncounted ages, a microphone was commonly called a mike. Some time during the last few years, everyone started spelling that short form mic. I don't know how that slipped by me. (It's my fault! I was asleep on guard duty!)
Why the change? It makes no sense. Mic should be pronounced mick, whereas mike is a sensible phonetic spelling. Did someone decree that mike was out out because the full word isn't spelled mikerophone? If that's the argument, then why don't people ride bics instead of bikes?
("Because if they did they'd risk getting burned in an awful place" is not an acceptable answer.)