Where do you get the flute on mythology island?

Go to the right of Main Street, to the Grove of Temples.

In the Temple of Muses in the Grove of Temples, you can get a free reed pipe (Flute) from the display to the far right. You play the notes by pushing the colored holes : blue, green, yellow, or red. Go to the statue of Euterpe, muse of music, and she will teach you the song to lull Cerberus. (blue-green-yellow-blue-red-green)

You will also need the reed pipe to enter the lair of the Minotaur (it has a musical lock that can be opened by playing blue-blue-green-yellow-yellow-red-blue-green).