I like how this sounds, but then again, I probably wouldn't

I did some experiments on digital data storage on cassettes a couple of years ago. Whatever the kind of data you want to store (compressed music, raw samples, etc) keep in mind that you are going to have a VERY low bitrate.

You can look into how digital data is normally stored in cassettes, and you'll find that they pick a frequency to represent "1" and one to represent "0". You would normally read at least 4 cycles at the slowest frequency to detect a one or zero, so your "baud rate" will be limited by how fast it would be to read 4 cycles of the slowest frequency you select.

Due to the frequency response of the magnetic substrate, a tape can't record anything above 16 KHz or thereabouts. And at that limit you are already getting a severely attenuated wave that will be hard to tell apart from noise. There are fancier, more expensive tapes with better frequency response, but the average tape was very limited.  So let's say you select 12KHz for "1", and 6 KHz for "zero" (which is already kinda pushing it).

Because you need 4 cycles of the lowest frequency (traditionally at least; you could try to get away with using less cycles), your highest baud rate in this scenario would be 6000/4 = 1500 bits per second. And that is without considering adding error correcting bits, or start/stop bits, etc.

Most computers from the 80s that used tapes would use lower frequencies (1.2KHz or 2.4 KHz at most, probably for reliability and to avoid getting close to the point where the tape starts attenuating the wave) and would end up with a baud rate of like 300 bits per second. 600 baud was fast, and 1200 baud was pretty much "turbo".

If I were you, I'd start by trying to figure out a format/compression that would allow me to represent a song at those low bitrates and check how that sounds, while finding a way to convert "normal" songs to that.

There is a good article on theis subject (almost) on Wikipedia:

https://en.wikipedia.org/wiki/Modem

It explains how you can get up to 56000 bits per second over a telephone quality audio channel.

I don't think that anyone ever optimised a codec for audio cassette data recording but since you can get 12kHz audio bandwidth and better than 45dB signal to noise ratio I would guess that  better than 100kbit/s is possible.

There is an article here

https://electronics.stackexchange.com/questions/596747/how-much-data-could-be-stored-on-a-standard-compact-cassette-using-modern-encodi

which discusses the subject froma  different point of view and concludes a (probably) lower rate.

MK

There is a good article on theis subject (almost) on Wikipedia:

https://en.wikipedia.org/wiki/Modem

It explains how you can get up to 56000 bits per second over a telephone quality audio channel.

I don't think that anyone ever optimised a codec for audio cassette data recording but since you can get 12kHz audio bandwidth and better than 45dB signal to noise ratio I would guess that  better than 100kbit/s is possible.

There is an article here

https://electronics.stackexchange.com/questions/596747/how-much-data-could-be-stored-on-a-standard-compact-cassette-using-modern-encodi

which discusses the subject froma  different point of view and concludes a (probably) lower rate.

MK

I've not read through the specifics of standard yet, but from a quick glance to the wiki description I assume they were using the amplitude of the wave as well as part of the encoding. Now, I'd assume that at that point encoding the data starts  becoming a lot harder