Welcome! 
If you mean the U-shaped thing that is in a central position on the top end of the display module, that is indeed the earspeaker. It should sound when you are having phone call in the traditional manner 
(i.e. holding the display side against your ear and cheek). When you play music or videos and the display is active, you should usually hear the external speaker instead (or more prominently) – at the bottom of the long slim side of the phone.
If you still think it might be defective, Fairphone OS offers a large range of hardware tests if you dial * # * # 6 6 # * # *
P.S.: If you like, feel free to say hello here.