1. Swipe up from the bottom while in the app.
2. Look for a circle containing a bell with a slash through it.
3. If it is highlighted, tap it to turn mute off, 
   → if you do not see the bell icon, go to step 4.
   
4. Flip the toggle switch on the side of the device above the volume keys.

   

Note: If your device is in a case, the switch may be hidden. Remove the device from the case to access the switch.