Improving sound effect is a broad concept, depending on how high you want to improve it. Adding speakers can effectively improve the sound field range of the whole vehicle. But it may not have much impact on the improvement of sound effects. Because the speaker as an output unit is an important element. A good speaker will have a good effect on audio restoration. General audiophiles may make more improvements to improve the acoustics of their cars. The first consideration may be to replace a better speaker, not to install it. It depends on your requirements for sound quality.