Tap the three parallel lines in the upper right to open the Options menu.
You can turn the game's Sound on or off with a tap.
If you have turned sounds on in the game but still cannot hear music or sound effects while playing, check that your device volume is set to an audible level and that your device is not muted.
Note for iPhone, iPad, or iPod Touch users: There is a switch on the side of the device, just beside the volume controls. If that switch is set to mute your device, it will override all other volume settings.