Sound issues could happen due to a wide variety of reasons. Please try the following solutions and see if they work for you:
Check the ringer switch on the external side of your device (iPhone/iPad)
Check the media volume of your device
Make sure that any "do not disturb" modes are not switched on
If you don't have an account and you're playing using a local player, try turning off WiFi and Cellular Data when playing (this resolves the issue in 90% of the cases)
Force-quit the app, then open it again if the sound stops - on both Android and iOS, please make sure you swipe the app away from the app switcher
If you have any music, video, or voice/video call apps open, please force quit these before opening the game. These include music streaming services, video streaming services, websites with videos or ads playing, communication apps like face time, Skype, Messenger, WhatsApp (etc.), mostly if you have recently used them for a voice or video call.
If you've read and completed all of these steps and you're continuing to experience sound problems, let us know: