Sound waves travel in the air. In fact, it is the contact between adjacent air molecules and the constant transmission of force that affects the movement of distant air molecules. The speed at which this effect propagates is the speed of sound waves. For example, one second can affect air molecules beyond 340 meters per second.
But what if these molecules move 40 meters against the sound wave?
This seems to be a synthesis of motion. This should be the Doppler effect.