I actually do know how to find where the music is but when I try to over write the current music with the new music its usually comes out all messed up.
The way I find the music is by opening the original NSF in an emulator with a Hex Editor.
Then match the values in the NSF and find the exact strings in the ROM, Since the NSF is basically a watered down version of the original ROM, The numbers will be the same.
Once I find the matching values in the ROM, I then take the values in the custom NSF and attempt to copy and paste them over the music in the original Rom. But....Apparently the music and the "TOC" are in separate places.
So replacing the music alone doesn't cause it to play correctly unless the game knows how to play the custom music. That's usually where Optomon comes in. Lol.
The music in Castlevania III starts at Address 100000, I find most games do.
Also, The music usually starts after a chunk of values that look like this... FF FF FF FF FF FF FF FF FF FF FF
I believe FF is a space filler between codes and operations.