>>277
public YourDontDestroyObject : MonoBehavior {
public PlayBGM(int scene) {
// bra bra bra
}
}

// some new scene detection script
void Start() {
GameObject go = Find("/OurDontDestroyObjectName");
if(go == nil) {
// may be first scene
} else {
//
YourDontDestroyObject bgmController = go.GetComponent<YourDontDestroyObject>();
bgmController.PlayBGM(SCENE_INDEX);
}
}