I have the "Dracula Best" soundtrack from Konami, which contains the music from Castlevania 1, 2, and 3. I had always assumed that they "prettied" it up a bit from the original games, as the music has more instruments and depth than what is presented in the NES games.
I knew that the Japanese version of Castlevania III supposedly had better music, so I decided to boot it up yesterday.
Holy crap! The music sounds just like the CD! I can't believe how ripped off we were in North America. I did a side by side test with the US version and it's no contest. It makes the US music sound like beeps and boops.
The superior music is due to custom sound chips in the game itself, accessed by the Famicom via two pins on the cart connector. Two measly pins. Nintendo of America found room to add 10 pins for the useless external connector on the bottom and 4 pins for the 10NES lockout chip, but they couldn't squeeze two more pins to allow custom hardware in games?
Thanks, Nintendo.