Here are some troubleshooting solutions for you to try:
Press on the power button for 3-5 seconds until you see the bar to power it off then power it back on, tap back into the application and see if that corrects the problem. The restart will usually clear memory issues on the device. Power down again for a longer period of time if it does not work the first time.
If you are still having issues after the power cycle, then delete the game from your multitasking feature and start the game again.
If you still do not have sound after a restart then check the tab on the side of the device and make sure it is not showing red. If it is, move the selector on. Check the sound option in the game to see if it is set to ON. Press the volume up on the side of the device and make sure that it is not silent.
For iPad users:
When you go into "settings" and swipe your finger upwards on the screen, a bar shows up at the bottom. In this bar there is a "bell" symbol. If the symbol is white, it overrides all other settings and turns the iPad into mute.