1. A system for warning the approach of an emergency motor vehicle comprising: a warning signal transmitter constructed and arranged for mounting on an emergency motor vehicle, said transmitter including means for generating and transmitting a radio frequency warning signal; and a warning signal receiver constructed and arranged for mounting on a motor vehicle to be warned of the approach of the emergency motor vehicle, said receiver including means for detecting said radio frequency warning signal, means for comparing the intensity of said detected signal to a threshold signal of predetermined intensity, said means for comparing providing an ON switching signal only when the intensity of said detected signal exceeds the predetermined intensity of said threshold signal, means for generating an audible frequency tone similar to a siren sound of the emergency motor vehicle, means for emitting the audible frequency tone within the passenger compartment of said vehicle to be warned only when said ON switching signal is provided, means for varying the intensity of said audible frequency tone in proportion to the intensity of said detected signal, and means for disabling selected sound producing sources within said passenger compartment when said ON switching signal is provided.