While staying in a high-end D.C. area hotel, I once discovered a hidden hard-wired speaker under the bathroom sink. Somebody had written "F.B.I." on it with nail polish.

I already knew that speakers could be used as microphones, and it occurred to me that putting a speaker in a hotel room in the name of "safety" would be a great cover story for a surveillance operation.

As a kid behind the Iron Curtain I had a radio & cassette tape player with a single large speaker built-in. The cassette player had a record button that when pressed together with play used the speaker and I could actually record my voice, and play it back, from the tape. After enough re-recording on a single tape the fade of the old recordings broke through the new ones creating a truly amazing sound experience. Too bad I lost the tapes and the device.

not only can electromagnet voice coils (speakers or microphones), be used as either a speaker or a microphone (transducer), they can even be used as both at the same time using a DC-coupled circulator / isolator:

https://techlib.com/files/rfdesign3.pdf

When I did commercial fishing in Alaska, often the boats just had two speakers, one in the wheel house and one on the deck (long liner). You just talked into/toward the speaker.

I used my moms Koss headphones as a microphone on her old stereo and used it to broadcast cast my teenage stupidity to the whole house.

A big speaker element is useful for micing kick drums.

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

That's all true. As a kid I was watching with sister a children's show in TV. The actor on the screen asked a question followed by silence. My sister yelled the answer at the TV, then the actor said something like "you're right kid". I was flabbergasted.

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