Technically, all the stuff heard at the beginning was initialization and training stuff. The actual data is the uniform hiss/roar just before the speaker cut off. https://www.theatlantic.com/technology/archive/2012/06/the-m...

At least for modems near the end of the period. The very first ones at 300bps really did sound like cassette audio.

Very similar to the sound of the Commodore data tapes when inserted to a regular cassette player. Tried it as a kid, I was convinced I broke something.

Random example: https://www.youtube.com/watch?v=wVsY9PVIKsQ

That was your modem negotiating the protocol and speed with the remote modem, not proper data. Still, if your mother happened to use the phone line while you were connected, she would heard the actual data on the line, just before the connection broke. Oh, the times.

You may like this classic HN post "The sound of the dialup, pictured" http://www.windytan.com/2012/11/the-sound-of-dialup-pictured... (HN discussions https://news.ycombinator.com/item?id=15635144 (675 points | Nov 6, 2017 | 108 comments) and https://news.ycombinator.com/item?id=9171514 (174 points | on Mar 9, 2015 | 35 comments))

modem = MOdulator + DEModulator

Dial-up modems used the public switched telephone network to transmit data via audio signals through regular phone calls. The modems had an internal speaker so the operator could “debug” the connection with their ears, e.g. if they called the wrong number and an actual person was talking on the other end.