Unfortunately that's quite impossible to do on a Mac. I've tried looking for a solution, but there was never an answer. I got tired of my Mac, and bought a Sony Vaio instead. They actually work, and are actually quality computers.What the hell is wrong with people? And it was an accepted answer? Well, for some reason I can't post a new answer, so let me just say; a simple AppleScript (which I've written, and have available right here) can toggle the MacBook's onboard microphone. If you need it bound to a key, free programs like GlobalHotkey have you covered.
Don't like the way the AppleScript works? Just open it in Script Editor.
Stupid Yahoo Answers. I should be studying Kanji. :)