SPEAKE(a)R: Turn Speakers to Microphones for Fun and Profit [pdf] (2017)

Some DJs use this principle when they need a hacky stage mic. They plug their headphones to the mixer's mic input, and shout to the speaker element.

Not all speakers work well as dynamic mics; and in fact turning on mic mode may enable the bias voltage, which could either burn out the voice coil or hold the diaphragm against the stop, making it even less likely to pick up any sound.

Jack retasking, although documented in applicable technical specifications, is not well-known, as was mentioned by the Linux audio developer

This could be a "bubble effect"; the Realtek codecs mentioned have a Windows utility to configure the jacks, which countless otherwise non-technical users would've seen and interacted with, so awareness of this feature is probably higher than they think. Fun fact: the "ALC" prefix in their codec names stands for Avance Logic, which was acquired by Realtek and they just kept that prefix well into the HD Audio era.

I have vague memories of iPod Linux (or Rockbox, I can’t remember) having a feature where you could record voice notes using your regular headphones using the same technique

Tbh it's crazy that you can do it in some of the microwaves

A magnet in a coil operates both ways, this is non intuitive but perfectly sound.

Not sure if it's mentioned in the article but microphones can be speakers too...

don't let the CIA see this one

That’s actually a pretty wild concept—turning speakers into microphones sounds like one of those “this shouldn’t work, but it does” kind of hacks

Okay, but how do I use this as a replacement when the mic is not working on Linux?

This needs a (2017), I was so confused why this was published again, seemed so familiar.

If this or an accelerometer based recording is what Meta uses to eavesdrop on in-person talk then color me pink