In case you ever wanted to build an own microphone, you should definitely take ribbon microphones into consideration. The basic elements of a ribbon microphone are two permanent magnets, a metal ribbon mounted between them, and a transformer (i. e. a ferrite core and some wires). The main challenges with getting it work properly are:
This is what I was able to build without buying any special parts for this project:
The metal ring is form an old hard drive, the NdFeB magnets are from an unused miniature motor, the ribbon and also the mounting of the magnets are made with sticky copper tape.
This thing really works (but not well at all). Here is a sound sample (with a lot of noise):
The main problem with this design is that the copper ribbon is just too thick and heavy to pick up sound effectively (a better idea is to use thin aluminum), so the sensitivity is the worst. Anyways, building a more appropriate ribbon microphone from scratch will be an interesting thing to try in the future.
You find more infos on the principles of ribbon microphones in their Wikipedia article.