There are several solutions:
1. Go to the game settings and make sure that the sounds and background music are activated.
2. If all parameters are set and the sound still doesn't return, restart the game.
3. If it still does not work, restart your device.
Generally speaking, these solutions work. If not :
1. Make sure your device is not in silent mode and the volume is high enough.
2. Try plugging and unplugging your headphones.
3. Make sure the music player on your device is turned off.
If nothing works, open another application to make sure your sound is working in general. If the problem persists, it probably comes from your device rather than the game.