I assume you're trying the CD version, so there is this post on Reddit. But I also recommend to get the standard version (Castlevania AGA HDF which is probably the simplest on RetroArch, because I've read that WHDLoad and LHA versions need extra libs). To get CD music going on emulation is really tricky, not all emulators have full support for CD games and I have experience only with WinUAE and FS-UAE. The standard version uses Amiga PCM chip music instead and should work without hassle. Thanks for the message and have a great Sunday!