AFAIK, the Famicom and NES versions of Castlevania's OST are identical (I can't seem to find any examples of it on YT, d'oh!). I know the FDS allowed for slightly "better" sound due to an extra sound channel, as is the case with Legend of Zelda, but as far as the first CV game goes, it's the same.
The big difference is with CVIII, which had the VRC6 chip embedded into it, which allowed its OST to be leaps and bounds better than the NES version.