Some extra footnotes for the above video that I didn't think were worth including:

1/ To get the music working with the "don't preload" mode, a small sub-heap is allocated inside the main one for... reasons. It works fine if the songs are just allocated to the main heap, so... eh? 2/ If you insist on keeping the DMA channel the same, there were many other, simpler ways to get this working. Like... why not just make the bosses load their own music? 3/ This whole "do stuff based on the background" seems to have been a dev favourite way of determining things, as a number of other things is tied to it. For example, whether the moving stair graphics should be loaded into Ghost House levels is ALSO tied to the background.

Uploaded to YouTube by Skawo.