Since I've been doing some hack work with Castlevania's music format, I finally finished up the old (preliminary) document I did describing its format:

Link

It's pretty thorough, and should allow you to do just about anything you'd want with the music, aside from instrument/ASDR editing; I may update it at a later date with that, but probably not... I'm primarily using the expanded VRC6 channels, so I'm not worried about that at the moment (I can define my own instrument sets with it).