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

Next, on iOS 12 and above, 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.

Next, on iOS 11 or lower, please try swiping up from the bottom 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.