What's mic-checking?
It's a tactic unique to the Occupy movement, also sometimes referred to with terms like "the human megaphone". It was developed in response to the original Occupy camp in New York being denied the permit required for any technological means of voice amplification (so no microphone/speaker setups or megaphones).
Somebody yells "mic check" and the crowd repeats "mic check" in response to let the person know that the crowd is recognizing them as having speaking power. From that point forward, that one person speaks in short phrases, which are repeated by anyone in the crowd who can hear him.
This technique gives speakers the ability to address large crowds without any equipment, and it's also been adapted as a tactic for public conflict at non-OWS speaking events. One other great advantage is it doesn't require any preparation. It's a part of the culture now and can be used spontaneously whenever needed. One great example was immediately after the infamous pepper spraying incident at UC Davis. It looked like a fight was about to break out, with hundreds of Occupiers screaming shame on the handful of bewildered police, until one guy could be heard above the crowd a few times screaming "mic check". Within seconds, the crowd was silent and motionless, except for repeating the words initiated by this one individual, who told the police (paraphrasing) that no one would prevent them from leaving if they did so peacefully. If you want to see the "mic check" thing at its best, that's the video to look up.