I hacked on this a while ago, and the best libraries I found were one called Wad for playing notes and applying effects, and another called Teoria for building chords and intervals and so on.

I was hindered by not having any idea what I was doing but here's what I came up with:

http://www.aphall.com/random/audio-test/ (clicks weirdly in firefox, no idea why.)

Libraries:

Wad: https://github.com/rserota/wad

Teoria: https://github.com/saebekassebil/teoria

This is clojurescript but maybe it'll be of some use to your mate https://github.com/ctford/klangmeister