Collect all five hidden Tori birds found in the following songs by pressing Y: World 1-4, World 2-6, World 3-7, World 5-8 (unlocked after completing the game), and World 7-3. To find them, search for a "cloud" portion on the ground. At the end of that portion you will find an egg. Hit it to get the Tori bird. Once you get all the Tori birds, return to Symphony City and unlock the door near the city to access Sky Roost.