Air Microphone is a US$0.99 app that uses AirPlay to connect your iPhone to an Apple TV or AirPlay-equipped amplifier to give you a wireless microphone.
The app is not hard to use. After launch, go to the multitasking bar on the iPhone and select any AirPlay device and start talking. In my tests, the app worked pretty well on an AppleTV with a few caveats. There are options for three EQ settings. "Normal" was just fine. "Less Bass" made me sound like a parrot, and "More Bass" made me sound like a giant Cyclops. Way too thundering, if you get my drift. The app also lets you select a photo to display on the Apple TV during your talk, but the image is proportioned like the iPhone screen in portrait mode, so I don't think it adds too much. Better to leave that feature turned off. The app has an "On Air" slide switch which turns the microphone on and off, but I could not find an elegant way to exit the app. Quitting leaves it running with a bar at the top of the screen. I finally just quit in the task manager.
Air Microphone is a very good idea, but it still seems a bit rough to me. I tried using an external Bluetooth microphone paired to my iPhone, but I could not get it to be output by the app. That step would make this app all the more useful.
Air Microphone is a very clever idea for an app, especially for those who do presentations. AirPlay is turning up on all sorts of non-Apple products, so there should be more and more options to use it in the venue of your choice. It works just fine on the normal settings with my AppleTV. I don't have any other AirPlay-ready devices to test it on. I'd really like to see this app advance in functionality. It has a lot of promise.
Air Microphone is optimized for the iPhone 5 and requires iOS 6.1 or later.
Update: Several users are reporting that they can't get the app to work with AirPlay based speakers. As noted in my review I was not able to test the app on anything but an AppleTV. I did request clarification from the developer in Europe but have not heard anything back. At this point, don't assume the app works with anything other than AppleTV.