Даже не сами
Re-encode it.
Either wrong codec used, or audio stream before video stream.
Go here:
https://video.online-convert.com/convert-to-mp4
Upload video
You can even make it so its compressed that it fits into 15 mb in cases the video is slightly larger. Don't do it for really large videos. You can cut these up, using Lossless Video Cutter.