Unlock Sound Mode and Time Attack:
To unlock Sound mode and Time Attack beat the game.

Unlock Zone X in Time Attack:
Successfully beat the game to unlock this extra level which has four acts and is not featured in the normal gameplay mode.

Unlock Blaze the Cat:
Go to World 3 and defeat the Kraken boss in the Coral Cave and Blaze the cat will become unlocked as a playable character.

Unlock Extra Zone (Super Sonic and Super Blaze):
In Sonic's story collect ALL 7 Chaos Emeralds and finish his story then complete Blaze's story. When both these requirements are met the Extra Zone will be available at the 'Character Selection' screen.

Unlock Tracks in Sound Test:
Tracks 110 through 127 will be missing when you first unlock sound test and to be able to unlock these you have to beat the Extra Zone. To beat the Extra Zone you have to complete both Sonic and Blaze's stories and collect ALL of the Chaos Emeralds as Sonic. If completed correctly a third icon will apear on the character select screen.

Unlock Extra Ending:
Collect ALL the Sol Emeralds and Chaos Emeralds to unlock Deep Core which is the final extra boss stage.