Get all 24 cosmo bubbles from all 7 worlds in story mode to unlock the Jukebox (sound test). After you unlock it you can buy it from the shop for 10000pts.
Unlock game credits
Defeat the world 7 boss and watch the ending to enable the credits to be purchased in the shop 500pts.
Unlock World 8
To access the eighth and final planet simply collect all 21 cosmo bubbles from the 1st seven planets.