Most of the time, when someone has audio issues with playback on a mobile device, it is because their mute button is enabled. Before troubleshooting any further, make sure this slider is disabled.
When you go to check your mute button (the physical slider on the side of the device), it may be 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.