hmm yeah, replaying the game now myself, i've found at least two other songs that are used in multiple areas at different pitches. the song for the snow world is also used in the "corridor" area of neon world with neon tiles at a lower pitch, and the music used in the ancient world where you find the 'hair' monsters is also used (at a lower pitch) in the dark bay / pier area, the train depot area before going down the escalator uses a slower version of the music in the warriors room (where you find the cat effect), the 'reception' area of the train depot has the same music as the title screen but, again, slowed down.
i think some of my name suggestions were not correct, too. the piano solo is not used in the spaceship for instance, it's used in the boiler room area on mars, and i think it is again at a different pitch.
there may be more. i suggest replaying it to hear it all yourself though.... there are probably a lot of things that can only be understood from playing the game.
