I started butchering the insides of a cassette a while back. I’ve used one of those cassettes you would put in a car to listen to stuff from a CD player etc. I’ve wired a piezo mic to the magnetic tape head that is normally used to transmit the audio from the CD player into the tape player. That amplifies everything that goes on mechanically in the tape. From there, I have replaced the main cog and started to attempt to build in a small mechanical musical instrument, which plays a different looping refrain when played in either direction.