Blaze the Cat
     Defeat the Kraken boss in Coral Cave (World 3) to unlock Blaze as
a playable character.

Extra Ending
     Deep  Core  (Final extra boss stage) Collect all Sol Emeralds and
Chaos Emeralds.

Muzy (Sound Test)
     After  defeating the Titan boss in Big Swell, go to Seagull Beach
on Windmill Island and talk to the koala with the guitar. You will now
be  able to listen to music, sound effects and dialogue from the game,
and can receive missions which unlock extra sounds once completed.