Playback sound of mov files.

I created a presentation with voice audio for each slide with Keynote. I then exported the presentation to an mov file. When I play it back on my iPad Pro 12.9-inch, 3rd generation, the sound is noticeably lowere for the first few seconds. On the other hand the play back volume is normal on my 11-inch iPad Pro and on my Windows 10 PC.

I cannot find anything in settings that addresses this problem or corrects it. I have seen similar complaints on various websites.

Does any know the cause of the problem and the solution. Below are my export settings. I used H.264 because that is most compatible with non-Apple devices for my friends.

Thanks