First of all, please try flicking the switch next to the physical volume controls, if your device has one.

 

Next, try swiping down from the top-right corner of the screen. That should reveal some additional controls, including a volume bar. Please make sure that isn't at the bottom. On iPads you should also find a 'bell' icon, please make sure that ISN'T highlighted.

 

If none of that helps, fully power off your device, power back on, and then try running the game again.