I just want to inform people who are claiming that the issue is resolved because of removing the protective cover, you are totally mistaken guys! I'll let you know why!
Because the main ISSUE is the sound drops down when the BASS volume in the recorded sound is going too high! (low frequencies I mean)
And why you think the issue is resolved, that's because you just recorded videos with no bass or the bass volume is not high.
***Try it yourself record a video for a song with high bass, then record the same song but cut or reduce the bass volume using the EQ. you will notice the difference***
SO PLEASE APPLE TEAM ADVISE WHAT IS THE SOLUTION, BECAUSE THIS IS VERY DISAPPOINTING, YOU RUINED MY VIDEOS GUYS!