The US soundtrack had an amazing end of level jingle that should really have been used in the JP version.
The rest of that music isn't all that great, though because the whole past <-> present/futures dissonance really ruins it for me.
However, most of the US music does sound quite good on its own and could probably fit better into a different game considering how it generally feels more like western 90s PC music (and I do like quite a lot of those).
Like, really. The US Present/Future versions of everything but Palmtree Panic and Tidal Tempest could have easily fit into a game related to western sci-fi on the PC.
But it is not suited for Sonic CD. It is as if you would play MM8BDM with the music from Duke Nukem or Doom, it would just feel out of place (but not necessarily sound bad).
The JP/PAL soundtrack on the other hand is really really good and fits to all of the stages perfectly, especially Tidal Tempest and Collision Chaos are just plain amazing in all 4 time zones.
It is probably one of the 5 best soundtracks in all of Sonic history.
And the boss theme is too good for words. It manages to fit to the tension of a boss fight while being incredibly silly and I don't recall having seen or heard something anywhere like it in any other game.
And not to forget, the game was supposed to have PCM samples to make the loops completely seamless in the JP/PAL version, but unfortunately that functionality was not implemented. More about that here (http://info.sonicretro.org/Game_Secrets:Sonic_the_Hedgehog_CD#Prototype_music_tracks).
// Feel free to stop reading here, the text below is only a rant about a single act in the entire game.
But the JP/PAL version has one really bad piece of music in my opinion and that would be Stardust Speedway Bad Future.
If it was an easy thing to do I would just replace it with the US version which is actually surprisingly good.
But really, the JP/PAL Metal Sonic race is just so incredibly unthreatening, silly and not serious because the music is not even remotely intimidating and actually full of annoying voice and sound samples. And no, this is NOT negating what I said about the JP/PAL boss theme. I really do like that one and it actually feels appropriate for fighting a boss.
The US version at least contains some tension because of the guitars, and the voices in the music support the whole decisive fight against Metal Sonic atmosphere.
Even the Sonic 4 Episode 2 theme of the White Park Act 4 fight with all the horrible instruments is better than the JP/PAL CD version in my opinion, while the theme of the Death Egg mk.II Metal Sonic race is for some reason even worse than the JP/PAL CD one.
Collision Chaos Past is my favorite piece of music of the whole game, by the way.
Tidal Tempest Present US is awful though.