Need help with AudioSources

You would need a "Audio$$anonymous$$anager" class referencing a list of secondary audiosource and a main audiosource (for the background music).

then in an update or a coroutine check if any of the the secondary audiosource is playing (using AudioSource.isPlaying).

something like :

 using UnityEngine;
 using System.Collections;
 
 public class Audio$$anonymous$$anager : $$anonymous$$onoBehaviour {
 
 
     public AudioSource[] secondarySource; 
     public AudioSource mainSource;
     private bool secondayIsPlaying;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
         foreach (var source in secondarySource) {
             if(source.isPlaying){
 
                 secondayIsPlaying = true;
                 break;
             }else{
 
                 secondayIsPlaying = false;
             }
 
         }
 
         if (secondayIsPlaying) {
 
             mainSource.volume = 0.5f;
 
         } else {
 
             mainSource.volume = 1f;
         }
     }
 }
 

This is just to get you started, It has not been tested. I also suggest a fade in/out animation ins$$anonymous$$d of a brutal change of the volume :)