Don't reload the AudioSource when back to scene

I guess the problem here is that when you load the scene again, this will never be the same as instance, because it's technically speaking a different object. Also the object that's kept loaded will not receive the Awake() call!

I would try it like this:

 static Object instance = null;
 
 void Awake()
 {
     if (instance != null)
     {
         Destroy (this.gameObject);
         return;
     }
     else
     {
         instance = this;
         DontDestroyOnLoad (this.gameObject);
     }
 }
 
 void OnDestroy()
 {
     if (instance == this)
     {
         instance = null;
     }
 }