1. Check the game settings to make sure sounds and music are turned on.
  2. If all the settings are fine and the sound is still off, try restarting the game app.
  3. If that doesn't help, try restarting your device.

That usually fixes the sound problem. If not, try the following:
1. Make sure your device is not muted and that the volume is turned up high enough.
2. Try plugging in and unplugging your headphones.
3. Make sure your music player is turned off.

If none of that worked, open another application to make sure your sound is working in general. If the problem persists, it probably has to do with your device itself rather than the game.