Here I'll list all the things I find that relates the different games's music, for example all the hints in Hyuponia etc. Feel free to add yours.
These are the reference videos I'll be using so that 1:00 mins are really 1:00 mins where I want.https://www.youtube.com/v/cfGRZPbkdUYhttps://www.youtube.com/v/drkUZB13ai8
I noticed that if you make the chord in song of rebirth at 1:08 minor, it is the same exact chord in Hyuponia
Hyuponia parts that hint or are from other game music
00:50 - Sad forest drums backwards (Klonoa)
1:27 - Lolo's theme (Klonoa 2)
I don't have the time right now to list everything so comment with what you found so that I'll add it in this list. I also want to analyze mirage (the forgotten path music). I'm starting to think that there are relations in all the game's music, for example many times the music in the first game sounded familiar going forward each level, Klonoa 2 had the same feel.