Fist open your Facebook app and go to it's settings, in it's settings look for the notifications section and see if everything is in order there or not (that is turn up the notification along with their sounds) once done, go to your phone's settings and go to the Settings menu>Sound. Once there turn up the notification sounds to whatever tone you like. That will give you a sound whenever Facebook pops up a notification (usually the first step, enabling it from the FB app is enough)