Here's Mobile Microphone in action - there's only the one screen, of course:
Skeuomorphism alive and well at Pico Brothers! Nice job, too. It's ad-supported, explaining its free status.
Interestingly, the developers point out that this works well with devices that have an FM transmitter too, so you can speak into your phone and have your voice transmitted out into the ether and re-amplified by FM radios within range.
This is a novel idea and it's hard to work out why noone has thought of it before. It's true that there's a very slight delay (a fraction of a second) between speaking and having your audio emerge from the 3.5mm port (or FM transmitter) but then the application's having to capture slices of audio and batch them up, so this is inevitable. And, with this very tiny caveat, I proclaim this a must have utility for the 'Oh no, our microphone's packed up' moment at a public event....
You can download Mobile Microphone for free here in the Nokia Store.