The Volume settings allow you to change the in-game volume.
Show Volume
Adjust the volume of background music, sound effects, and character voices during shows. The higher the number, the louder the volume.
System Volume
Adjust the volume of background music, sound effects, and character voices outside of shows. The higher the number, the louder the volume.
Note that the volume settings on your device also affect the game.
If you cannot hear any sound from the game, check the volume settings on your device, and make sure it's not in silent mode.