At the title screen (while BGM is playing) press the following buttons: L, R and Select. Note: the lyrics will appear after about 14 seconds.
Secret Characters (use the enemy characters)
To play as these characters first hold both L and R at the character select screen. While these buttons are pressed you can freely move the selection cursor around the screen. Select a specific part of the screen (read the Secret Character Guide FAQ for more details) and press Y, A or B to play as the character. NOTE: this also works in 2 player mode. also, while using this code, the player 2 can choose the same character as the player 1 (or vice versa).
Sound Test
Go to the Option mode, move the cursor to EXIT, and then press START while holding LR and SELECT.