Excellent tutorial. Back in the early 90s, LucasArts used this sort of technology called “iMuse” in Monkey Island and other games, reworking the song to transition to the next rooms audio. This has been something i’ve been wanting to do in my game, so I’m happy to see someone did it before me! 
Thank you so much.
I think this can be taken even further. I wonder if we can work it a certain way where we have music separated by instrument, fading/volume/pitch depending on the trigger, more so like iMuse. Its an interesting thought, but it would greatly increase the size of the final game.