If sound is being played by other apps, such as a music app, the in-game sounds may not play.
Please close all other running apps, and then try restarting the game.
If the above does not apply to your issue, please check the following.
- From an in-game screen, tap Menu (the squares and diamond icon in the top right corner of the screen) > Options, and then check the volume for sounds such as BGM.
- Check your device's volume settings in case it is muted.
- Turn off your device, then turn it on again and restart the game.