[snailmailman]

I definitely had some cheap PC speakers in the mid 2000s that worked as a shitty microphone if you plugged the cable into the microphone port instead of the speaker port. You basically had to yell into them, and the sound quality was atrocious, but it did ‘work’.

A surprising amount of circuits work like this. Generators and motors are basically the same as well, and spinning a fan by hand will induce current into the wire. I assume most circuits are designed to properly handle that happening, but I still worry when blowing the dust out of my PC and will manually prevent the fans from spinning when I blow compressed air at them.

[atoav]

Okay that must have been speakers that were driven directly by the line signal? Otherwise you'd expect an amplifier inbetween the line input and the speakers.

This intuition isn't true anymore since today many speakers will have "smart" in them and MEMS microphone ICs are a thing.