Doing stuff like this in the 8-bit era was never particularly hard. The difficulty was doing this and then anything else, because you basically need the entire CPU just for audio synthesis. For games, there was a very real trade-off between calculating video, game state, and sound, and sound often took the back seat.

It's also why everything looked and sounded better on 8-bit computers that had hardware support for graphic sprites or sound generation, even if they had a less powerful CPU. And it's the untold story behind why 16-bit graphics looked so much better: it wasn't about the CPU as much as it was about having a dedicated video card / chipset with its own memory, a sound card that handled polyphonic sound and MIDI synthesis, etc.

I have a memory of a Spectrum game playing some sort of synthesised speech through the speaker that seemed incredibly ahead of its time. I believe the game was called iBall. And the sample said something like “Fight iBall!” Have I misremembered that? Was this a fever dream?

Due to all the constraints the ZX Spectrum is a fantastic system for recreational programming; too bad I missed the latest Spectrum Next batch last year.

This reminded me of a recent very detailed video on polyphony implementation on 8052

https://news.ycombinator.com/item?id=49895486

There's also a phone with YM2149

There was a music editor I remember getting on a late Your Sinclair edition cover tape which displayed proper music staves and gave you 4-note chord output. Blew my mind a bit at the time.

And who could possibly forget Chronos' title music, by Tim Follin?

https://www.youtube.com/watch?v=u-D24A_N4d4

ZX's audio and video systems are two things that I never missed.