Tap the menu icon in the upper right, and open the Settings menu.
From there, you can toggle 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.
On many devices, there is a switch on the side, typically located near the power or volume controls. If that switch is set to mute your device, it will override all other volume settings.