MV: For context, the scene I mention below comes from Toy Story 2. The toys are attempting to cross a busy freeway, hidden under traffic cones. These cones cause the cars to swerve, resulting in a crash, leading to a pile-up. Will the toys make it across unscathed? Or will they get caught in the incurring chaos unfolding around them?
This scene, like many, works because of the contrast. You switch from moments inside the toy world to the much larger environment of the car's and freeway. Contrasting these worlds provides the emotional rollercoaster that the audience experiences. Will the toys complete their voyage? Or will the unknown occupants of the cars impede their mission?
You bring a scene to life with contrasting elements, or by giving it a dramatic shape with dynamics. Think about taking the audience on an ark, telling a story throughout each scene with sound.
Sound designers may create something that takes advance of surround sound and moves between the speakers.
The music is there to emphasise emotions, which in this scene, is tension and danger. It lets the audience know what is happening and how they should respond. In this scene, the music underscores the toys coming alive, but when they stop, the music ceases, and the car's world returns.
When it comes to a final mix, you have multiple options. Such as:
Where and how to deploy the sound effects.
Where does the audience need to hear the dialogue? How should this inform where you can place the sound effects?
Where does the music need to drive the scene, and where can it sit underneath the sound effects or dialogue?