A recording done with just the mic cable, without a mic

Funny you'd bring that up. I have one cheaper guitar cable that I'm sometimes forced to use. It's a noisey thing. If I step on it I can clearly hear it. lol. When I pick it up to move it I can hear many sounds, not just the one you describe. The gain is very high of course. Could I shout at it and hear myself? I wouldn't be supprised. Lol. So yes, I've heard the effect before many times. You can hear your fingernail plinking against the glass of a preamplifier tube in a highgain circuit too. In that case, the filiments vibrate and act like a mic. As for the cable; I guess we're generating a current by squeezing the atoms in the cable? And generating a current. Similar to piezo element? Can't imagine what else it'd be. A good test for bad cables? Maybe? Not sure. Jim P. .

Someone smarter than me may correct me, but as I understand it it's not a piezo effect, but rather an capacitance effect (not that I understand it much further than that). As it was explained to me, that's why more expensive low-capacitance cables are worth the extra money. I've had cheap guitar cables that did this (including lot of diy cables), but my Mogami cables do not.

Invalid test, the balanced nature of the cable requires it to be terminated at both ends. Opening one end of the cable defeats the circuits ability to achieve it's "CMRR": common mode rejection ratio, whereas signals/currents induced into the circuit are cancelled due to their relative phase.