Hi, not sure if you're talking about standard version or CD version. Standard version is incomplete atm, music-wise.
However, the CD version can be modded, so you can download your favorite Castlevania tracks and do a replace-and-rename job in its folder to create your own custom playlist. The current track list is described in this post.
It supports MP3, WAV and FLAC formats (but you have to edit the CUE file accordingly, check screenshot below). If you use emulator, make sure to load it by the cue file, otherwise it'll ignore the CD music. If you burn a CD to play on real console, make sure that the burner you use supports the music format that you gonna use (people recommend ISO Burner which is free, and to burn at slowest speeds).
Thanks!
