Rob,
The Mic's on these locomotives are outside the cab (at least on UP locomotives) to record the Bell and Whistle usage in case of a lawsuit by an errant driver. There have been many "You didn't whistle for the crossing so I'm gonna sue" cases settled quickly with these videos. These cameras are not mandatory but are being put on locomotives by the legal or "risk management" department. This is not like a "Black Box" flight recorder. What you were hearing with the brakes was the exhaust from under the floor, probably next to the bell.
Stop and think about the reputation that Railroaders have for language. If there was a mic in the cab the video probably would have to have an "Over 18" password attached to it.
Rick