Well I got the cassettes out and have been fiddling with recording off them, not done yet. I found one cassette with some basic programs on it for an Imsai 8080 (life, star trek, etc), and a piece of paper with a track listing, so I can tell which program is which! Basic programs will be pretty useful as a test, as even if it won't run on the basic you have on your s100 machine, it should still be readable as ascii.
I can get the sounds into my pc, and am experimenting with amplifying the signal via the audacity sound editor program (it's pretty weak off the cassette player), seems to work pretty well. I should have something for you tomorrow night (pacific US time).
I'll actually not save them in mp3 (lossy) format, but in some lossless format. Since audacity runs on windows, mac, and linux, I'll save the files in FLAC; I know audacity can write and read that on all platforms. Then you can for sure use audacity to play back the audio to your computer's speaker out and plug a cable from there to a cassette tape recorder and recreate the cassette tape.
It's been pretty fun listening to the sounds, takes me back to the old days. I only used tapes for a little while, but the sounds are similar to a slow modem. I can hear bits in my headphone!
Jeff