public class dontStopAudio : MonoBehaviour {

    void Awake()
    {
        GameObject[] objs = GameObject.FindGameObjectsWithTag("BGAudio");
        if (objs.Length > 1)
            Destroy(this.gameObject);

        DontDestroyOnLoad(this.gameObject);
    }

}

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class toggleMusic : MonoBehaviour {

    public GameObject bgMusic;
    public AudioSource bgAudio;
    public GameObject toggleMusicButton;
    public Sprite soundON, soundOFF;


    public void bgMusicOnOff()
    {
        bgAudio = bgMusic.GetComponent<AudioSource>();
        if (bgAudio.mute = false) {
            bgAudio.mute = true;
            toggleMusicButton.GetComponent<Image>().sprite = soundOFF;
        }
        else if (bgAudio.mute = true) {
            bgAudio.mute = false;
            toggleMusicButton.GetComponent<Image>().sprite = soundOFF;
        }
    }
}