I wonder how hard it would be to write my own airgap audio transmission program to send transactions between computers.
Back in the good ol' Trash80 & TI 99/4A days, storage was done not on USB, nor HDD, nor even floppy. We used FSK (frequency shift keying) to encode our ones and zeros as audio tones, and stored it on analog cassette. And we _liked_ it.
No reason you couldn't use FSK across the speaker/mic interface.
Now get off my lawn!
Damn juvenile delinquents....