Why not just play the audio on a computer speaker to a microphone and maybe a generic audio preamp board ( to get the levels right ) to the destination computer.

Could have the speaker and microphone in a small cardboard box to avoid any audio interference ....

This happened to me in the early 80's with the family's Tandy Color Computer 1. We didn't have a disk drive, just loaded things using tape drive.

As an aside, I think the modulation used was a simple frequency shift keying, with likely no error correction.