Alright, my interests have shifted. Instead of a Castlevania III ROM hack, I've found something far more simple to do.
I want to prevent all the music in Mega Man (World) II on the Game Boy from playing, but leave the sound effects active and functional. This isn't for a hack or even sound effect ripping. I actually just want to mute the soundtrack so I can make a playthrough where I overlay a completely different soundtrack over the semi-silent game. Silly idea, I know. But, it passes the time.
In any case, I've found that the music is stored at $1CCC0 to - $1F442 (with three trailing $00 bytes, for some reason) and that tinkering with the first byte -- $F0 -- will make the game play the New Weapon tune all throughout the game non-stop. Messing with the first three bytes will also silence the game music, but the sound engine will eventually deteriorate and cause the sound effects to stop working properly. That being said, I have no idea where to go from here.
If anyone wants to tinker with it, I'd be very appreciative. Otherwise, don't sweat it. I'm just goofing around, after all.