I can really recommend a copy of Nicolas Collins’ Handmade Electronic Music, which touches on similar topics, and elements of this level of simplicity.
What seems to be going on in that box, visually, is:
- there’s a contact mic going into some kind of contact mic amp. This is powered from the 9V supply.
- there’s some acoustic sound-making things - strings, springs - and a motor fed from the supply, possibly with a pot in the way to alter its speed.
- there’s a simple amp of some kind and a speaker.
so really, most of it is your standard passive box, with one/two extra components. If you’re looking for a headphone amp circuit, the CMoy circuit is simple and works really well. To amplify a contact mic from a unipolar power supply like a battery, the Alex Rice circuit is good.
I appreciate that’s not a ton of detail, but the Collins book starts from the point of view of ‘a contact mic’ and goes from there, focusing on battery circuits; it’s a lovely thing to have anyway. And then, at the other end, you’re making something out of building blocks: a couple of distinct units (mic amp, speaker/phones amp, connections, perhaps a motor circuit), which you can build up in turn.