Maybe one could perform accesses on mechanical hard drives in such a way you could encode a signal with it. Maybe reading a couple bytes from the inner tracks o

Maybe one could perform accesses on mechanical hard drives in such a way you could encode a signal with it. Maybe reading a couple bytes from the inner tracks of the disk for "0" and outer tracks for "1", assuming they produce different but consistent access sounds. Reading certain areas of a floppy disk would probably be even louder and more consistent. There could be a "setup program" that listens to the computers microphone and performs various accesses, choosing two that meet the criteria for being distinguishable enough for another system to tell apart. Lower frequencies for 0, higher for 1.