There is a very interesting article about Interactive music and its use in Wwise in Music4Games:
Making Music Interactive: Elaboration of the Feature Set in Wwise
http://www.music4games.net/Features_Display.aspx?id=317
http://www.music4games.net/Index.aspx

It is sort of fun that we have to invent new language to talk about interactive music. And it brings up questions like how to charge for interactive music.