Laroy Tymes, who was then a machine operator, wrote a program that gained considerable fame inside and outside the Lab. It was Stars and Stripes Forever, for the CDC 3600. It used the speaker attached to high order bits of the program counter to provide one or two voices, the noisy Analex line printer to provide percussion and for a tuba like effect the windows of the tape drives would be opened a bit. The starting and stopping of the tape was under direct millisecond control of the software and the vacuum columns (designed to provide mechanical buffering of the tape) provided excellent sound coupling.
There is another story here of obscure provenance. Some system designers were offended that people would use the programmable indicator lights to perform music. They recognized that speaker output was a useful indicator of normal application behavior and included a speaker driven by just a few bits in the program counter. The 3600 designers were supposed to have thought that this would thwart music programs. Instead this simplified the music program for to control the sound signal was merely a very fast branch. This allowed an early form of two part music. I don’t recall whether Laroy’s program used this trick.