That’s being played through a piezoelectric buzzer that’s being rapidly flipped on and off to do pulse width modulation. But that’s not the clever bit: the clever bit is how the different tracks are being mixed together.
I have an Acorn A7000 to fix. The CMOS battery has leaked horribly and, even if it hadn’t, it needs to be replaced.
I’m slightly surprised the A7000 is still supported!
Testing in production is a superpower. It’s our inability to acknowledge it that’s the trouble.