The Electronic Music System (or E.M.S. for short) took close to eight years to reach version 7.03. During that time, Sean managed to cram just about every feature he felt a decent music driver needed into the code, as well as functions added at the suggestion of the other Cosine members.
Previous versions of E.M.S. was used to produce the soundtracks for commercial games including Elvira - Mistress of the Dark and Turn 'n' Burn by Flairsoft, Ninja Rabbits by MicroValue and (probably most famously) System 3's excellent Turbocharge where the in-game soundtracks from Marc and Sean use just two channels to simulate three, leaving one free to supply sound effects without losing any of the music. It's also been used extensively for Cosine demos from Lethargy onwards.