Unlock Dark World:
When you have beat ALL the A worlds in the Light world with DEV enter the following code at that title screen to unlock Dark World. A, Left, Right, A.

Unlock Evil Sorcerer:
Select 1-A, then 2-A, ALL the way to 5-A in Puzzle mode and when the last puzzle in 5-A is completed you'll get the code to visit 'Another World'. After the DS restarts the Evil Sorcerer will be unlocked as a playable character.

Unlock Sound Test:
At the Main menu screen enter Select, A, B, Left, Right, A, Select, Right.