I know that I'm missing a few themes according to the full ost, but this is what I managed to find.

Ground Theme

https://www.youtube.com/watch?v=TPZmLTVl1jY

Introduction Theme

https://www.youtube.com/watch?v=yPMd5x6B0Kw

Snail Theme

https://www.youtube.com/watch?v=hqeD6QQa0sM

Plotline Theme

https://www.youtube.com/watch?v=5B-MwK4rYwE

Underground Theme

https://www.youtube.com/watch?v=BzWTyR6ykbE

Bonus Level Theme

https://www.youtube.com/watch?v=RE8kx7N0TEY

Final Step Level Theme

https://www.youtube.com/watch?v=5ToUUSNY11I

Ground Theme 2

https://www.youtube.com/watch?v=VpVJ7XNCvQc

Final Level Theme

https://www.youtube.com/watch?v=PEX75aVNr2E

Boss Theme

https://www.youtube.com/watch?v=YSJcR61es7k

House Theme

https://www.youtube.com/watch?v=XcFkV7VSvek

Squiggleville Theme

https://www.youtube.com/watch?v=81I1oWCx6vQ

Outer Squiggleville Theme

https://www.youtube.com/watch?v=D4dR-5ZRff8

Pirate Cove

https://www.youtube.com/watch?v=45-v9eOscqs

Pirate Ship

https://www.youtube.com/watch?v=z6A9yk0Lq6o

Royal Tub

https://www.youtube.com/watch?v=am5D_HMIoWw

Disco Room

https://www.youtube.com/watch?v=10HfszcTy8I

Back to the Video Game Music Archive

Which one is your favorite?