You should be able to simulate the Atari cassette recorder's both tracks (audio and digital data) with a CD-player by recording the audio on one of the channels (say left channel) and the digital data on the other channel (say right channel). Then you just need to feed the audio out left channel into the SIO audio in line and the audio out right channel needs to be converted (and amplified) from 0.. 1Vpp to 0..5V DC and fed into the SIO Data In line. You could record the CD in such a way so that the audio playback rate (150kb/second) produces a 600bps equivalent data output in the conversion circuit for the data and audio normally (on the left channel). Then you could use the MOTOR control line to set CD to start playing current track and DATA OUT line to seek next track in a loop through all tracks on the CD. Some programs may need to be modified to use MOTOR/DATA OUT properly. For new custom programs, they can use the SYNC marks put on the CD (via location 53775) and take advantage of the CD features for audio w/o loading any programs from it. The hardware should work with any CD player with stereo audio outputs.
I guess if you play the mono CDs on a stereo speaker system, the right channel would just cause some clicks at end of audio tracks or phrases.
Yeah, this would be a fun little project. I suppose it's also be possible to play wave files off of an SD card or something like this, but I kind of see the retro-factor in having a physical CD version of some of those old Atari tapes. It provides a straightforwards way to backup -- audio to audio.
Me, I never actually owned any of the educational System tapes. I assume when they reach the end, they prompt you to rewind? I guess there's no way to detect this, and the player just has to push a button to restart the thing. Or when the Atari goes 'Bleep, bleep -- motor on' then it knows to rewind? And just 'motor on' means continue on the current tape?