[mattnewport]

It would be much more accurate to say these objects are neither speakers nor microphones. It is not a surprise that all objects vibrate in response to sound waves. What characterizes speakers or microphones is that they are transducers converting between acoustic signals and electrical signals. None of these objects are transducers so it's not accurate to call them speakers or microphones. It is reasonable to refer to the system of camera plus software plus observed object a kind of microphone but certainly not to refer to the observed objects as speakers or microphones.

An aeroplane flies through the air and a submarine travels through the ocean. I can throw a rock into the air or into the ocean, that doesn't make a rock an aeroplane or a submarine.