Firstly can you check that you have not accidentally muted your phone as follows: https://support.apple.com/en-gb/HT208353

Secondly verify that both music and SFX options in the game are turned up. You can access these from the settings menu (tap on the cog icon at the top right hand side of your screen). The sound options are under "Preferences" which is the third screen along (tap the arrows to change screens).

Lastly check Apple's advice on this topic: https://support.apple.com/en-gb/HT203794

If the problem persists, do not hesitate to contact the support service and give them more details on the issue. Does it happen with headphones or with the speakers, do you have any apps running in the background? Do other apps work with full sound? They will need this information before investigating your issue.