The basic trick is that it's using a pentatonic scale. So the output will sound more or less harmonic regardless of the combination of tones.

So, here's a little 128 byte beeper demo I made for Outline a few weeks ago. Basically what it does is play back the Spectrum's ROM as a 3-channel beeper tune, and also produce some glitch visuals on the side. Unsurprisingly, it couldn't really compete with the all the mindblowing stuff that other people made for the compo. But considering it was my first 128 byte prod, I'm nontheless proud of it

You can download it here, or watch it on youtube.