It seems like DriveCam only records the sounds within the vehicle. So, unless your drivers have passengers, there should be no problem (assuming they have all consented to having their voices recorded). From the looks of this sample, the technology seems effective even without audio - unless you are trying to prove your daughter listens s to the radio too loud !
This observation is provided without warranty nor guarantee and for entertainment and informational purposes only. This answer is not legal advice. Not to be used as infant formula. No attorney client relationship is established as a result of these communications. The best legal advice you can get is to consult with an attorney licensed in your state or territory.