One of the things I wanted to do, that wasn’t directly related to the boss’ behavior, was to have the BGM switch to something more intense when it spawns in.

Passing in the desired AudioSource and AudioClip allows this method to be used for any BGM switching that needs to happen.
Step one is to fade the current track out so that the switch isn’t so abrupt.

After the current song is muted the AudioSource is stopped, switched to a different clip, and played again.