After beating the game, save your game and it'll be your save clear file. Start a new game on the save clear file and you'll find your costumes and accessories are brought over from your previous game. Two new accessories will be added to your accessories list, the Spike Batch and Spike Sign.
Spike Batch - Play a new game on your save clear file.
Spike Sign - Play a new game on your save clear file.
Sound Mode
After beating the game and saving your clear data, go to the main menu and click on extras. Now Sound Mode will be available.