Beat the game to unlock this option under the "Special" menu.
To get the complete ending, beat Graham and get all of the ancient books. You will then fight chaos. Beat him to get the full ending.
To play on Hard mode, beat the game with the full ending.
Beat the game normally (with Soma), then start a new game and enter "Julius" as the name.
Enter "NOUSE" as your name at the beginning of the game.
Enter "NOSOUL" as your name at the beginning of the game.
To unlock the sound test mode, just beat the game once with the full ending.
