A free program for Android, by VB Applications.
Phone Mic - Use Phone as Mic for Loudspeakers is a free and easy-to-use app for converting your mobile phone into a microphone. It can be used in a number of different situations, such as when you have to make a speech in a large hall or when you are going to give a presentation. In these situations, the microphone will be the most important part of the device. The app allows you to choose from the presets like Jazz, Pop, Rock, and many others.
The app also supports the Bluetooth and mobile jacks. You can use the app as a microphone in any situation where you need to use your microphone.
