If you required the user to download a plug-in, or if you used Flash on the same page where your game is. Also, you can use x-webkit-speech, which doesn't post the mic but only the words spoken into the mic: http://slides.html5rocks.com/#speech-input.
Unfortunately, this is no good for what I want to do. I want to post the volume of the mic. That way, I can make a game with a warrior who swings his sword dealing more attack damage when the summoner(s) yell louder. For example, +1.5% attack damage per decibel of sound.