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

I guess the first task is to find the tape recorder. From memory you will need to read the file back into memory first before it can be played back. The data rate is very slow. 

If you want an idea of the circuitry that was originally used in these kind of interfaces, this is a cassette tape interface I built around 1979. I think it might have been based on a Don Lancaster design. It worked at 300 baud and was quite solid and reliable recording and playing back. The large chip is a UART. The panel switch is labelled play and record. I think that the relay controlled the motion of the tape, somehow, maybe via the small mains PSU I built to power the recorder to save on batteries.

Having read through all the replies so far, I feel you might be interested in 'Packet Radio', a system of data transfer between two computers using a tranceiver each (on the same radio frequency) and a modem usually based around a Z80. Zero's and One's were harmonically-related audio ferqencies. The system would work with several users on each channel, it coped with 'collision detection' and slightly inaccurate baud rates. If more than four zeroes were sent then a system called 'bit-stuffing' was used to insert a One into the data stream to re-align the clocks, which were phase-locked to the incoming signal. The receiver would remove it so that the data was not corrupted. The dual tones were audio frequencies and used the CUTS tape interface on the Beeb. This also selected Transmit or Receive using the relay contacts that normally stopped and started the cassette recorder.

Reading your contribution made me think that using a similar system you could use multiple recorders to produce Electronic Music based on random mixing of several different cassettes set to playback, simultaneously and/or only one at a time, or any combination of the above. There is an article on Wikipedia that explains it all far better than I can, it must be 40 years since I used packet radio! 

jc2048 Thank you for the reference, I believe you are referring to Don Lancaster’s “Bit Boffer”

www.tinaja.com/.../bitboff.pdf

It's very possible, though I don't now recollect the article. I didn't get to see Byte much until later - the various magazines weren't imported here in the UK very much at that stage, but I did obtain from somewhere the first book where they rolled together a lot of the more interesting content, so perhaps it was from that. I might still have it - I'll have a rummage around later.

The only US magazine I read regularly was Dr Dobbs, which I had a subscription to for a few years (it got flown over air mail). For a while, that had some really good stuff in it - I can remember rewriting one of the various Tiny BASICs that appeared, from 8080 code to 6502. Indeed that might have been part of my motivation for doing the cassette interface: it would have been painful entering that in machine code every time I wanted to use it.

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

He included the 'Bit Boffer' in his TV Typewriter Cookbook. So that's where I came across it.

Would it work if stanto  simply took the analogue bits (with maybe an actual comparator on the input to the receiver, rather than using the 3130 op amp) and attached them to a Pico? Could the PIOs emulate what the CMOS logic is doing and the program emulate the UART? If so, that might be the simplest and cheapest way to do what is being asked for.

You built it in 1979, I just finished high school and you still have the book. Impressive or horder, not sure on that? Actually I know I still have my college text books from that year. My missus keeps asking me to throw them out.

I threw out the missus 

OK, I guess it's about time I trim my library. I hereby resolve to throw out my 1980's textbooks before the end of the year. For sure that will be worth lots of points from my beloved.