How can I load a new scene without losing context of previous?

I'm making a car game and I want to allow the player to:

1. Go to a new scene to select a different car (the previous scene should be paused)

2. Return to the previous scene without losing context, replacing the car with the newly selected car.

I've tried this:

 using UnityEngine;
 using UnityEngine.SceneManagement;
 using System.Collections;
 
 public class CarSelect : MonoBehaviour {
 
     public void LoadCarSelectScene() {
         SceneManager.LoadScene ("CarSelect", LoadSceneMode.Additive);
     }
 
     public void UnLoadCarSelectScene() {
         SceneManager.UnloadScene ("CarSelect");
     }
 }

LoadCarSelectScene is triggered via a button click.

What I'm seeing is that the next scene is not loaded and I get the following warning: There are 2 audio listeners in the scene. Please ensure there is always exactly one audio listener in the scene.

This seems like common functionality for many games, but I'm struggling to find out how to do this.