imovie video is out of sync

HI,

I have just completed a video with iMovie. At the beginning of it there is 20 secs of speaking then music begins. Ive uploaded it to both YouTube and camera roll and its out of sync. On camera roll The speaking starts about five secs later but corrects itself later in the video when the second piece of speaking is in sync. The upload to YouTube is out of sync throughout. I'm using the iPad pro but I remember having this problem when I had an iPad Air.

How can I correct this?

THanks