Nikon D4 audio "fade in" porblem, how to report to nikon

hi hi:

I have some problem shooting video with Nikon D4.

I saw the sound level is normal, when I press the rec button.the sound with drop Suddenly.

you can see the attach file, all of the sound clip have this problem.

have anyone like this? how can I ask Nikon to fix it?

Chung