As you enter the island there will be a stone head. Lift it up with your new power bracelet. Then enter the cave. The wall ahead has inscriptions. Use those to play a new song. Take out your Wind Waker and play the notes. This teaches you the Earth God's Lyrics. It allows you to warp to the Earth Temple. Then a Zora sage comes out and tells you that a harp player is required to open the temple. The only harp player that we know of his Medli. So head back to the Dragon Roost Island. Warping there is the easiest way.