That microphone is used by the Hands Free phone and TeleAid system. I am not sure if it is also used for ambient noise. There may be other microphones located in other places as well.
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Law-makers are supposed to regulate in the interest of the public as opposed to regulating in the interest of the regulated party.
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Ambient noise mic for what? As far as I know, as stated above, the mic is only for the phone and TeleAid systems. As for the stereo - if that is why you are asking - the volume rises and lowers according to speed. It has nothing to do with ambient noise.