hi , i just posted this for guy using galaxy ace 2 , it's much the same for u.
after you done whats below , open whatsapp , go to setting and notification , notification tone ...
i have galaxy ace ( not 2 ) lol , go to your " MY FILES " folder and create a new folder and name it notifications , any tones you want for messages etc put in this folder.. when in messaging go to settings and scroll to notification settings , select tone and you should see the tones that are in that folder ..