Jun Senoue on Twitter:
Fan: Actually, the US version did have the same Past music for every zone. It was the Present and Future music that was.
Jun Senoue: Yeah, past music was programmed ones with internal sound chip of Mega-CD. It was easy to replace the CD-DA music.
Fan: Cool little tidbit there. The past music doesn’t fit too well with the US version though. I prefer the JP soundtrack more.
Jun Senoue: Technically it’s very easy to replace the music, but it isn’t possible to understand “how to design” its game audio.