No sound on iOS

Development Audio
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1

I use the MoviePlayer to show a loading screen.
The .mp4 video is played without sound.
Then I Create a new project based on the FirstPerson C++ template, then pack it to iOS, it have no sound too.

2

I installed the package on another iPhone, it’s OK.
Still not work on my iPad air 2, ios version: 11.4.1

3

Check to see if silent mode slider is turn on

4

If it works with headphones but not phone speakers there’s a good chance silent mode is just on. Don’t know the work around in the code settings to make it play without silent mode on sadly.