On the title screen says hold down the following keys, A, B, LEFT, UP, SELECT and press START. Use LEFT, RIGHT, UP or DOWN to select a tune and press B to play.
If it's done right, a sound test screen with a Chibi Ryu will appear. To exit the sound test screen, press START.
Now try this, let the title screen fade to the intro and then press START to return to the title screen, you'll see the title screen 2 times, use the button combination again and a sound test screen with a Chibi Irene appears.
To exit the sound test screen, press START.
Now that's not it, even another sound test screen is available. Let the title screen fade to the intro, the press START to return to the title screen, let it fade again and press START one more time
, you'll see the title screen 3 times, then use the button combination and if it's done correctly you'll be taken to a screen where both Chibu Ryu and Irene are present. This is the ultimate sound test screen while the other two only plays music,
all sounds are available here.