Yes, me and a friend from the robotics club at school tried it the other year (everyone else in the room was pretty annoyed) it works but it's very finicky:

Your sound card is going to work best in some small band of frequencies (around 3khz if I remember) the mark and space tones have to be pretty far apart and the volume has to be turned up pretty high.

Turning the baud rate up increases your error rate so unless you're including error correction in your transmission that has to be pretty low.

My guess is that the soundcard works pretty uniformly well between 100Hz and 15kHz. But the speakers and microphones might not.

Yup, you can run a speaker cable between mic/line-in and speaker/line-out to pretty good effect. When I'm working on radio protocols I usually use this if I don't feel like setting up a pair of radios.

If you want to try that with a different modem in your browser, check out https://quiet.github.io/quiet-js

Yes