There is an adjustment in the maintenance section of the nav radio to turn up the gain on the microphone. Chances are that you are not speaking loud enough, or there is too much ambient noise in the cabin.

Adjusting the microphone gain sensitivity will fix this. (or you can just speak louder)

The microphone might also be picking up harmonic noise from the chassis. The happens when the microphone comes dislodged from its isolation mount and touches a hard point on the bezel it is mounted on.

You can have your dealer set the gain on the microphone a bit louder and or check the microphone mounting.