Check this out: (this is where I got the idea of recording through speakers)
In 8th grade, I took apart a dynamic microphone I had picked up from radio shack. It was essentially a tiny speaker! Naturally, I went ahead and connected it to the headphone jack of a portable cassette recorder and...
Heard some of the BEST sound I had ever heard out of small electronics! It was not loud, but it was detailed and full so long as one held the mic right against the ear.
I bought another one and mounted both of them inside some hearing protection type ear covers, essentially turning them into a sort of weird headphone.
Later on, a friend showed me a Walkman type radio / cassette player and the little open air type speakers it came with looked a whole lot like the dynamic mic "speakers" did and had a similar sound too.
I had come close to producing open air type listening gear on my own. My first attempt at stereo listening failed because I just did not have a great head mount. Went with the ear protection gear because that fit well and did make the most of the great, but somewhat quiet sound the little plastic cones in the cheapo dynamic mic could deliver.
Had I been able to come up with a suitable mount, it was extremely likely I would have ended up with two small speakers resting right on my ears Walkman style.
In any case, that is when I realized speakers can be microphones and that lead up to the point of discussion here now.
Speakers and a certain kind of microphone are exactly the same thing at the basic level. Sound hitting the diaphram (the bit that moves to produce sound) of a speaker will cause a voltage to be produced on the speaker wires that can be recorded.
Most speakers are basically a linear motor: there's a coil (the stator) and a diaphragm with iron in the middle (the "rotor"). When you apply an AC waveform to the coil, it moves the diaphragm in and out, producing sound. But it also works in reverse: moving the diaphragm in and out with sound produces a voltage in the coil. So fundamentally, a speaker and a microphone are really the same thing, just wired differently (and usually scaled differently: speakers are typically larger, and microphones typically have tiny diaphragms to be more sensitive).
This concept is quite general: most sensors and actuators are transducers: i.e. they work in both directions as a sensor and an actuator. It varies from one to the other how well something designed for one role can work as another, but in the care of speakers they are actually reasonably good microphones.
Another fun example: solar panels are LEDs and LEDs are solar panels. You can detect light with an LED (and in theory power something from it, but you'd need a lot of light and not need much power) and you can emit light with a solar panel. Most solar panels will only emit light in the infrared, though. I have seen a triple-junction solar panel being back-driven by others in parallel with it, and it glowed a deep red, which is pretty cool.
Most types of speaker are a microphone, they just need to be wired up correctly (hence the mention of a "capable preamp") to be used as one.
For example, a piezoelectric device that deforms when driven by an electric current produces a little current when deformed by external forces.
I'm sure there are many more examples but famously Groove Armada recorded the trombone parts of "Superstylin'" and "At the River" through a speaker because they didn't have a microphone in the studio.
Ok, these guys are great! I am watching the superstylin video right now! Lol, that near futuristic handheld locator, and their over the top, lightly overdrive, "in your face" production style hit solid.
That trombone part: well, it worked out well for them. There is one quiet part where we can hear the horn pretty well and I have to say the response of the speaker rolled off a lot of the subtlety of the instrument.
Normally, that would be bad, but for the overall production style of the tune, it just works! Feels a whole lot like a sample ripped from a cassette, or lo-fi vinyl setup.
