A device for automatically controlling audio volume based on vehicle speed and a method for operating the same keep the car audio volume at a uniform level relative to noise occurring at a level proportional to vehicle speed. The device includes a vehicle speed calculation unit, a car audio device including a speaker, a mode-setting button, and a microcomputer. The mode-setting button is formed on the car audio device at a portion thereof so as to allow a user to set an automatic volume control mode of the car audio device. The microcomputer automatically controls volume of the car audio device so as to output an audio volume corresponding to vehicle speed calculated in the vehicle calculation unit, according to the automatic volume control mode set through the mode-setting button.