It's a simple speaker that can be built at home from inexpensive and freely available materials: magnet, cardboard and tape, isolated wire from signal transformer and mini-jack cable from broken headphones.
Above is a sample taken from this speaker. In real it's really quiet, but its sound quality is pretty good ;)
An homemade inductor with ~80 wire loops was connected it into mini-jack cable.
The inductor was attached to the center of a cardboard circle.
The circle was placed on a magnet.
You can also check my other article, where I presented how to make a simple homemade microphone.