In the Pirate Fortress, there is a penguin in the eastern platform. Follow it and it will lead you to a hidden screen with the last pipot and a chest containing Cubic Music Score 9. Bring it to any Cubic Musician inside the White Boa, Numara and Uhra and play:

1 - Rest, E, E, Rest
2 - D, Rest, C, D
3 - E, G, A, B
4 - F, E, D, E
5 - B, A, G, E
6 - C, C, D, E
7 - A, Rest, E, D

Your reward will be the Sunderch Sunglasses that will enable you to find the hidden chests.