You can not successfully use the camera mic while trying this as you'll gt no usable sound. You will need a microphone in the vehicle that is connected to the camera via a cable or wireless.
Besides the audio, there is always a risk when mounting a camera on a car. Murphy's law you know. I would worry more about rocks, debris, dust, ect. If driving in regular traffic you may also, attract the attention of diligent law enforcment types.