when people in the front of a car talk just loud enough that they can hear one another but too quietly for those in the back to hear. effect is added to by music in a car generally being louder when sitting in the back.
Dave: Hey Fred, how was your car journey here?
Fred: Absolute crap, Matt and Greg sat in the front and used front seat voices all the way here, I couldn't hear a thing!
Mum: How are we going to discuss directions to the theatre without my mother being a back seat driver?
Dad: Don't worry, just turn up the radio and use front seat voices.