You think two channels is the limit? Have you heard title music from Chronos? Full multi-channel polyphony, without relying on sound co-processor. I'm not sure where to find the game itself these days, but I found the music itself here [0] on position 31.
https://spectrumgamesuk.com/index.php?game=Meteor-Storm
"...possibly the first game with speech synthesis, albeit sounding like an unintelligble gargle."
Quicksilva later improved their speech engine. QS Chess, for example, clearly said "This is the Chess Player".
And when I'm mentioning cassette recorder - I had a neighbour in my building who also had a Spectrum and whose window was some 12 meters away from my window and once we connected our computer with audio cables (MIC to EAR and vice versa) so one types some short program (or just REM message) and enters SAVE while the other one enters LOAD and gets the message. Our networking adventure started bad as it turned out that cable had large signal loss so it seemed that this brilliant idea failed but then we connected cassette recorders between computers that hugely amplified signal and thing actually worked.
Experiment ended when a party-line neighbor started complaining that his phone line was unusable for hours - we used phones as yelling through the window while trying to synchronize wasn't producing satisfactory results.
Fun times.
I've personally had a lot of fun repairing one I bought on catawiki.
Bought one at a garage sale for 5 EUR. I'm pretty sure it does not work. Haven't tried it: I still have a good old CRT TV so I could.
But when people talk about coding for these 8-bit machines, they usually mean they'll target an emulator.