Using headphones as microphones
Two of the sets of earbuds didn't work (due to a puppy's gnawing); one set of earbuds was uncomfortable for the kids; and the two headphones simply didn't work.
Most headphones and speakers work on the same principle as dynamic (moving-coil) microphones -- but in an opposite direction: headphones turn electricity into vibrating air; whereas microphones turn vibrating air into electricity. So, in a pinch you can use headphones and speakers as (somewhat inefficient) microphones.
The top photo shows the headphones that I tested, and two of the three sets of earbuds. I later found a third set of earbuds on my desk, which I had tossed there a few weeks earlier with the same intent of possibly re-purposing them as a microphone.
That is: if the puppy had torn off the right-side earbuds, and retained the left side -- then they probably would've worked as microphones.
I believe that the left-side earbuds (and headphone speakers) work as microphones is because of how the "one-eighths-inch to one-quarter-inch adaptor plug" will "ground" the plug -- i.e. combines the "ring" signal with the other portion of the plug.
And, this photo shows the finalists in this process: the two "real" headphones, plus the black "they work, but they're uncomfortable to wear" earbuds. All of them actually worked as microphones. So, three out of five isn't too bad.
I've posted a YouTube video below, that demonstrates the sound quality from these three "microphones". I used two signal sources: my voice, and a harmonica.
If I'd thought further, I would've also compared these to a "real" microphone, like a Shure SM57. Ah well.
The blue headphones seemed to be open-backed -- so on the voice test I also spoke into the back. Also notice that I often tried the mics from two positions: about six inches, and about a foot. Because I recorded these during my kids' piano lessons (occurring in the adjacent room), there's noticeably more "spill" with the more distant micing -- as one would expect.
I edited the audio clips together in Audacity (freeware multitrack audio software), and added some simple compression and normalization, just to make the various sound clips comparable and of similar volume. No equalization or other effects have been added.
So: pretty decent for "free" microphones.