Csound

Barry Vercoe wrote Csound at the MIT Media Lab in 1986, in C, as the latest link in a chain that began with Max Mathews' MUSIC programs at Bell Labs. The lineage shows in the structure: the orchestra defines instruments as chains of opcodes (oscil, reson, pvsanal), and the score lists which notes they play and when. These days both live in a single .csd file.

Originally everything rendered to disk, and a minute of audio could take all night. Today it runs in real time, accepts MIDI and OSC, and ships as a library for Python and the browser. Csound is probably the oldest sound language still under active maintenance, and has kept backward compatibility with a stubbornness few projects can match.