That's pretty strange! Never heard of this issue before...
I imagine that sound works on all the other apps on your device with no issues? Have you checked the options menu to see if your game has the volume turned all the way down? You also will need to take your device off silent in order to hear the game.
If none of that works, perhaps send us an email at skullgirls@hiddenvariable.com so that we can look into this further.