From Smudge Apps:
Megaphone is a must have app for your iPhone. Megaphone transforms your iPhone into an instant Microphone, PA system or Megaphone when you plug it into amplified speakers. It is really simple. If you are at a party and playing music from your iPhone on the stereo, you can just launch Megaphone, and use your iPhone as a Microphone as you make a speech. You will never need to hire a microphone again. Just plug in a stereo or amplified speakers into the headphone jack, launch Megaphone, and start talking. Its that easy. Plug the iPhone into an FM transmitter for a wireless microphone. Great for speeches at parties or functions. Full instructions provided.