Diagram: https://i.sstatic.net/8rSD2.jpg
This is basic physics controlling the effect here, not electrical routing. Speakers are microphones by their very design. To make them work as a microphone, you merely speak into them with them plugged into an input jack that provides at minimum a line level electrical signal to be modified by wiggling the speaker cone/diaphragm back and forth.
If you click "record" on your computer, there's no way to tell it to record signal from the speaker output channels, even if you write a custom low-level application directly making OS calls. The OS can't even do it, because it's not supported by the firmware.
Dynamic loudspeakers and dynamic microphones are the same thing. They always have been the same.
They've got the knobs for the design variables turned in different directions, but they're still the same.
They even have the same frequency response whether they're being used as speakers or microphones at the moment.
Which brings up a valid way to measure the response of a microphone's design:
Use two of them. One as a speaker, and the other as a microphone. Play measurement-sounds out of one, and record the results on the other. Plot it out.
The deviations are magnified, but eliminating that magnification is just a math problem -- not an instrumentation problem. :)
