Williams used the same sound hardware (and many of the same sound effects as well) on all their arcade and pinball games from the late 1970s through the mid 1980s. And unlike other machines that used dedicated sound-generating chips like the AY-3-8910, the Williams sound board was just a 6800 CPU hooked up to an 8-bit DAC. It did everything in software. No square wave generators or hardware timers. The pitch of the sound was controlled by variable-length delay loops!
I spent a bit of time reverse-engineering the sound ROMs a while ago. From what I recall, there were a few different sound-generation algorithms. Some of them generated simple pulse waves or noise. Another was wavetable-based; it used samples in ROM and manipulated them in various clever ways. Another one, which was responsible for their coolest sounds, used a bonkers system of multiple nested loops generating variable-width pulse waves... I was never able to fully understand it...