The esoteric programming language Velato uses music as its source code. The first note of a composition establishes a “command root” note, and the intervals that follow specify instructions. The command root can be changed between statements, and the notes that make up a chord can be interpreted in a specified order, so there’s some latitude to help a composition sound “musical.” This program produces the output “Hello, World”:

Here’s what that sounds like:

A few other musical languages: Fugue, VenetianScript, Yet Another Musical Esolang.