If your game has gone silent, there are plenty of potential fixes. 

Start by checking the in-game settings. Are your music/sfx set to “on?” They should be.

If your settings are in order, but your game’s still muted, restart by fully exiting the app, and reopening it.

If neither of these help, please try the following:
• Important: make sure the mute switch is set to OFF
• Try plugging in, and unplugging some earphones
• Open another app, and check if sound is working there. If not, then it probably has something to do with the device, and not the app itself