A vehicle has an engine and a sound amplifying system characterized by a frequency response including a signal source and a manually operable sound level setting control that establishes the sound level of the desired signal in the vehicle, a power amplifier and an electroacoustical transducer. The manually operable sound level setting control comprises a dynamic equalization circuit for providing a predetermined equalization over the bass frequency range depending on the sound level setting of the sound level setting control. A variable gain device associated with the sound level setting control responds to an input electrical audio signal to be amplified by the system for providing a signal of controlled amplitude. The variable gain device controls the gain of the system to increase and decrease in response to the engine being on and off respectively. The manually operable sound level setting control changes the frequency response of the system when altering the gain of the system much more significantly than when the variable gain device alters the gain such that the frequency response with the engine being on is always different from the frequency response with the engine being off.