No, the sound frequency of a car does not change for people on both sides of the vehicle if it is moving at a constant velocity relative to them.
Sound waves are produced by the car's engine and other sources, such as the tires on the road. These waves propagate in all directions, including toward the front and back of the vehicle. When the car is moving at a constant velocity, the sound waves propagate uniformly in all directions, and the frequency of the sound remains constant.
The frequency of a sound wave corresponds to the pitch or tone perceived by the human ear. It is determined by the source of the sound and does not change as long as the source remains constant. Therefore, if you're standing on either side of a car that is moving at a constant velocity, you will hear the same frequency of sound, assuming you are at the same distance from the source of the sound.
However, it's worth noting that if the car is approaching or receding from an observer, the frequency of the sound heard by that observer will be affected by the Doppler effect. As the car approaches, the sound waves are compressed, resulting in a higher perceived frequency (higher pitch). Conversely, as the car recedes, the sound waves are stretched, leading to a lower perceived frequency (lower pitch). This effect is noticeable when the car is moving toward or away from an observer, but it does not depend on the position of the observer relative to the sides of the vehicle.