So i was editing some video footage the other day, I had two sets of audio. one was the ingame sound of the game i was playing, the other was my voice. Now the one with my voice picked up a little bit of the game sound so I spent an hour or so syncing them together to make sure you couldnt hear an echo. i went through this entire thing and got it perfect. but for some reason, after uploading it to youtube, its unsynced and theres an echo. I made sure it was synced before I saved the project. Im using Movie Maker to edit. How the Heck did these track desync? and how do i keep it from happening again?

i save it as an mp4