First unlock the two secret stages by collecting all the dolls. Beat one to unlock Hurdy Durdy. This will allow you to hear any of the game's music. Beat the other to unlock more tracks for the Hurdy Durdy.
First collect all 150 gems on each stage. Then you will be rewarded with special images in the gallery.
Each stage has six stars. If you collect all six you'll gain a doll that will appear on the L, R screen. Collect eight dolls to unlock the first hidden level and another eight for the second level.