Most audio issues with playback on a mobile device are because the mute button is enabled. Before troubleshooting any further, make sure this slider is disabled.
Check your mute button (the physical slider on the side of the device), to see if it is in the muted position. Typically, when set to mute, it shows a little bit of neon orange. Also, when you first set it to mute, the device will vibrate once. So if you can't see the orange, switch it until you feel the vibrate, then switch it back the other way to be unmuted.
After you switch it, try playing the video again.