First, make sure you are on the latest game version and OS update for your device.
Press on the power button for 3-5 seconds until you see the bar to power it off. Slide the bar and power the device down 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. Press the 'Home' button twice real quickly, the screen will popup, press the Application icon and swipe your finger from the bottom to top to remove the item from multitasking. If there are many applications in there, you may want to clear them out as well. Then press the middle button to get out of the multitasking mode 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.