To list,
A dynamic microphone (coil and magnet) can be
used as a small speaker for the same reason that a generator
can be used as a motor. Bob Masta has noted that
a condenser microphone is similarly reversible. But
many inexpensive condenser microphones come with built-in
FET, and these are presumably not reversible because of the
asymmetry of the FET.
Best,
Bill H.