A friend (A) of mine was recently coerced into getting Skype by another friend (B). However, A did not have a microphone, and B told her to just go ahead and plug some headphones into the microphone jack and talk into them. She has apparently used this setup repeatedly and successfully.
At first I thought they were yanking my chain, but I have no reason to suspect they would try and pull some random prank over on me.
Has anyone else ever done this?
I’ve done it, and it works.
Yep, will work with almost any type of speaker, but the impedance may not be appropriate for the mic input, so often you gotta yell.
Microphones are really cheap now, but back in the old days kids walkie-talkies almost always used the speaker as the microphone, to shave a few pennies off the BOM.
In really secure technology/military installations, radios are prohibited.
It is very possible to transform a speaker into a microphone.
Standard microphones vibrate because the air pressure impulses from the sound are hitting them. They are attached to magnets which pulse inside coils of wire, which (IIRC) are magnetized by electrical current. Any conductor moving inside a magnetic field creates an electrical current. This current is then fed to an amplifier, which is fed to a PA system, or a radio station board, etc.
A speaker vibrates to make audio/air pressure. I don’t think I can do it personally, but it’s technically trivial to reverse the process so that the speaker cone vibrates as a result of air pressure, pushing the magnet up and down in the coil, which creates an electrical current as an analog of the speaker vibration. This current can be recorded or broadcast as an electrical signal.
What’s more, depending on the style of microphone, it may act as a speaker. The sound quality won’t be good and it can’t take a great deal of power, and it’ll sound very tinny, but it’ll work with some mikes. It worked with the cheap Radio Shack model I had, though it may not work with every kind.
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Yes, it can even work quite well; a while back, an assortment of dopers recorded some voice samples for me, to be used in radio jingles; some of them were recorded by plugging earphones into the microphone socket, and they were just fine - certainly comparable in quality with the ones recorded using actual microphones, but I suppose in terms of computer equipment, that isn’t very surprising; the speaker elements in your average pair of earphones is not all that different in quality or construction from the microphone element in your average computer microphone.
I remember one of the final scenes in Honey We Shrunk Ourselves. The Shrunken Adults do just this. I remember thinking at the time, NO WAY!
Well. apparently so.
On further thought, I think the microphone pulses a permanent magnet through a coil of wire, meaning that a magnetic field is cutting across a conductor. I don’t think the coil is magnetized.
Please correct if I’m wrong.
You’re in the ballpark. In practice, it’s the coil of wire that moves (because it’s much lighter than the magnet), but the effect is the same: a time-varying magnetic field across a conductor induces an electric current through it. This current, in turn, produces a magnetic field, in opposition to the external field.
On NPR they had an interview a guy had done in some country in Africa where he was prohibited from bringing in a microphone. He recorded the whole thing through the head phones. The sound quality wasn’t that great, but you could hear everything fine.
Yeah, that ruined the whole scene. Otherwise, the movie was totally believable. 
When I was in a high school band (back in the 70s), we wanted to record some of our practice sessions. We had a reel-to-reel tape recorder and some good quality Sennheiser headphones, but no microphone. Well, we tried putting the headphones on a boom in front of us and plugging them into the tape recorder. The results, particularly when played back through the headphones, was nothing short of spectacular! The sense of space and realism was unbelievable. The usual quiet chatter between songs was absolutely indistinguishable from someone standing nearby and talking.
I learned later that we had discovered “binaural” recording, where the recording is made with two microphones placed about 6 inches apart and intended to be listened to through headphones.