For a game project (Chime), I made a fairly simple adaptive music system in Unreal Editor's Kismet. Chime is a slow paced game, and the music had to evolve slowly as the game progressed. I made a system with 3 versions of the music playing simultaneously, but only one audible at a time. When a trigger was activated, the music would fade seamlessly to the new version. Check the video to hear the system in action.
The video shows an early build. For demonstration purposes, every time the player activates a music switch trigger, a "beep" is played as well.
The main first layer consists of some orchestral instruments, the second layer adds some soft choir sounds, and the third layer adds harp notes on top of that. Music made by Jeffrey van Rossum.