Play as 8-Bit Simon
After you've beaten the game and obtained Boss Rush Mode, input Up, Up, Down, Down, Left, Right, Left, Right, B, A, Select while the Konami logo is being shown. Enter Boss Rush mode and Simon will be automatically selected.
Unlock Boss Rush Mode
To unlock the Boss Rush mode, where you play every single boss again, one after another, simply beat the game.
Unlock Sound Mode
To unlock Sound Mode, where you can listen to all the music from the game, You must have all Dracula parts and you must wear Juste's and Maxim's ring when fighting Maxim in Castle B. Afterwards, Dracula will show up. Defeat him to get Sound Mode, which you can select at the main menu.