Dynamic microphone uses a moving coil around magnetic field to convert mechanical movement into electrical signal. The ordinary mini speaker is turned into a very sensitive microphone. Although it looks like misusing the speaker as a microphone, but actually a real standard dynamic microphone uses the same principle as this speaker.
We can use any NPN transistors such as BC 547. The circuit will operate from 2v to 9 v. It’s a common-base amplifier and accepts the low impedance of the speaker to produce a gain of more than 100. The best type for the speaker is the all-range type, almost any diameter could be used. The larger the speaker diameter the more sensitive the reception.