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. This is because sound is a wave that propagates through a medium, such as air, and its frequency is determined by the source of the sound.
When a car is moving at a constant velocity, the sound it produces is emitted from the source (e.g., the engine or exhaust) with a specific frequency. This frequency remains constant regardless of the car's velocity. As a result, people on both sides of the car will perceive the same sound frequency, assuming they are stationary relative to the car.
However, it is worth noting that if the car is moving toward or away from a stationary observer, the perceived pitch or frequency of the sound may change due to the Doppler effect. The Doppler effect is the apparent change in frequency of a wave when there is relative motion between the source of the wave and the observer. When the car is moving toward an observer, the sound waves are compressed, leading to a higher perceived frequency (higher pitch). Conversely, if the car is moving away from an observer, the sound waves are stretched, resulting in a lower perceived frequency (lower pitch). This change in frequency is known as the Doppler shift.
However, if the observer is also moving at the same velocity as the car, in the same direction, there would be no relative motion between the source and the observer, and the perceived sound frequency would remain constant for them as well.