Click the Lock button in the track header. If the Lock button isn't visible in the track header, choose Track > Track Header > Show Lock Button. The lock turns green to indicate that the track is locked. To unprotect the track, click the Lock button again. read more
). Click on it and it will turn green to indicate you want the track to be locked. read more
When you Lock a track, all the work of creating any instrument or rendering any effect is done at that time, and saved to disk as a new (higher resolution, 32-bit [Significantly larger]) file. When you play this locked track, all GarageBand has to do is read it from disc and play it. read more
The problem is, tat the .mp4 you are importing to GarageBAnd is added as a movie track, and that is behaving differently from regular tracks. You cannot shift the position of the audio of a movie track. read more