There are several possible causes for this issue, and you can try the solutions below. Number 3 below seems to do the trick for most people.
- Make sure the hardware mute toggle switch on the side of your device is not set to mute. The intro movies will play with mute on because they are media files (like a movie or music), but the in-game audio will not play if the mute toggle is set to mute.
- Make sure the volume settings in the game not set to 0. Check these from the main menu (or pause menu) in Options/Sound Options/.
- Try the solutions suggested on this web site (related to the hardware mute toggle switch).