Over the past couple days I wrote some scripts that convert data to audio, and audio back to data. I've tested it by recording it onto cassette tapes and after tweaking it, I've been able to record data onto tapes and read it back with 100% reliability, even with large files and repeated testing.
I'm using amplitude modulation, which seems to be uncommon with this kind of experiment. All the 80s home computers used frequency modulation for this, as far as I know.

https://github.com/dusthillresident/johnsonscript/tree/master/example_programs/data_on_audio_cassettes