Topic: Electronic toys, wristwatches, keychains, greeting cards, and so on.
Recent castpixel's post made me think that we kind of missed a whole big underground of 1-bit sounds and music. One that we can't really exploit due to the nature of the things (one time programmable, made to cost the least possible amount), but it is still interesting in a historical perspective.
In my own experience, in my childhood I was very impressed by a keychain with like 8 buttons, making various sounds effects and melodies through a small speaker. There were plenty of those, some of them responded to a whistle. Here is a video of one of them, very similar to one I recall. As I realize this might be one of the earliest examples of 1-bit synthesized sounds I ever heard.
Then there were wristwatches with many melodies. Many of Casio ones, also a very popular brand Montana. Here is a video.
Later there were many musical greeting cards, paper ones that start to play a song when you open it. Today they all fancy and play digitized audio, but early ones had just a piezo with a monophonic melody, very similar to the wristwatch. Turns out to be somehow difficult to find a video of those now.
I also recall a Russian IC to make door bells, I think yet another clone of something older and similar from the West, first appeared in early 90s. It also played a monophonic melody. In fact those chips are still made, in dozens of versions with different songs! Have no slightest idea who would need them now. Here is a video.
I think I also have heard something non monophonic and non square wave among these things, but can't remember it now, besides those Brick Game toys.
I don't know what exactly is inside those devices, but I suppose it is some 4-bit MCU, similar to the SM-510 or TMS1x00. Most of these devices use a 32768 Hz crystal, so it must be something more fancy than just a set of counters and a ROM (a hard logic implementation of a song player).