Well you have to go Single Player >>> Time Attack. Then choose any character, use it. When you win, there will be an unlock, read the description.

Example: you picked Naruto, and You passed all ten characters from ten battles, you will recieve an unlock from the sound player(after the game show you the time how fast you battle in 10 characters from 10 battles),it says : "You can now listen to Naruto's voice"

(I don't remember where I got the sound player. If you have a sound player, you can hear it)