Williams games, from the DEFENDER and ROBOTRON era played one sound at a time.
This code reproduces the Williams sound board, which ran a Moto 6800 at .85xx Mhz. It took arguments sent to it and or commands to play sound X via a parallel input. The classic Williams sound came from parameters input to this board. They would literally type them in and write down good ones.
Jarvis spoke about a sound priority system to give good players the right, most important sound at the right time. I hardly noticed! Read that and went to listen and defender actually interrupts sounds as needed! Which can actually make a new sound to the player. He has a GDC talk about all this.
I read somewhere in the past that they had a development system that they used to make the sounds that would power-on with random junk numbers in its memory. They would play the sound and make note of the good ones and power cycle to get more.
This web page is great stuff and I am happy to see it created by its author.