Unlockable Pokemon Songs:
Unlock the following songs by performing the corresponding tasks. To access the song, press Y and shift the song lists all the way to the left:

Bicycle Theme (Pokemon Gold/Silver) – Complete Calypso Beach (World 3).
Champion Theme (Pokemon Red/Blue) – Complete Sleighbell Mountains (World 5).
Pokemon Gym Theme – Complete Marching Hills (World 1).
Route 26 Theme (Pokemon Gold/Silver) – Complete Rock Range (World 2).
Trainer Battle Theme (Pokemon Black/White) – Complete Baroque Valley (World 7).

Sky Roost (World 8):
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.