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.