Someone on reddit wondered about part of OrbitLab’s tagline “listen to the music of the spheres”. It’s a good question!
According to the article in Wikipedia:
Musica universalis (lit. universal music, or music of the spheres) or Harmony of the Spheres is an ancient philosophical concept that regards proportions in the movements of celestial bodies—the Sun, Moon, and planets—as a form of musica (the Medieval Latin name for music). This ‘music’ is not usually thought to be literally audible, but a harmonic and/or mathematical and/or religious concept.
In OrbitLab, it’s a matter of playing a tone at the farthest and nearest points of an orbit (the apsides). The tones assigned are relative to the distance of the planet from the sun at those points. The net effect is something like a celestial wind chime, though it’s more periodic. And it depends on how many planets you have, of course. A single planet in OrbitLab will go forever just emitting two tones, possibly separated by many seconds.