It's kinda hard to describe but they sound the same.It's the same type of music. Orchestral sometimes, with the use of more modern instruments(electric guitar, drums...) I know that all those games have the same composer(Michiru Yamane).I find that her music has become very repetitive. Plus I think her masterpiece was SOTN, and every soundtrack she did for CV after that one was just a failed attempt to make something as good , with the exception of COD, LOI and Aria. OOE had good potential, but I'm not really convinced by some of the tracks.
Still I don't get why Konami decided to stick with one composer who did most of the music for the metroidvanias, while they could have hired different composers and have more variety. If you consider games that came out before SOTN, they didn't sound the same. The music in CVIV has nothing to do with the one used in Bloodlines or Dracula XX.Those had great soundtracks, and in a way I wish their composers would come back and make CV music again, or anyone who can make good music, as long as it fits the mood in CV.