MusicPlayer.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine.SceneManagement;
using UnityEngine;

public class MusicPlayer : MonoBehaviour
{

    static MusicPlayer instance = null;

    AudioSource audioSource;

    public LevelManager levelManager;

    public AudioClip splashTheme;
    public AudioClip mainMenuTheme;
    public AudioClip levelOneTheme;
    public AudioClip levelTwoTheme;
    public AudioClip levelThreeTheme;
    public AudioClip normalEndingTheme;
    public AudioClip settingsTheme;
    public AudioClip perfectEndingTheme;
    public AudioClip gameOverTheme;
    public AudioClip storyBookTheme;

    [HideInInspector]
    public int thisSceneIndex;
    [HideInInspector]
    public int lastSceneIndex;

    private AudioSource music;

    // Start is called before the first frame update
    void Start()
    {

        levelManager = FindObjectOfType<LevelManager>();

        DontDestroyOnLoad(this);
        audioSource = GetComponent<AudioSource>();
        audioSource.volume = PlayerPrefsController.GetMasterVolume();

        if (instance != null && instance != this)
        {
            Destroy(gameObject);
            print("Duplicate music player self-destructing!");
        }
        else
        {
            instance = this;
            GameObject.DontDestroyOnLoad(gameObject);
            music = GetComponent<AudioSource>();
            music.clip = splashTheme;
            music.loop = true;
            music.Play();
        }

        lastSceneIndex = SceneManager.GetActiveScene().buildIndex;

    }

    private void Update()
    {

        thisSceneIndex = SceneManager.GetActiveScene().buildIndex;

        if(thisSceneIndex != lastSceneIndex)
        {
            ChangeSong();
            lastSceneIndex = thisSceneIndex;
        }

    }

    public void SetVolume(float volume)
    {

        audioSource.volume = volume;

    }

    void ChangeSong()
    {

        if(levelManager.currentSceneIndex == 0)
        {
            music.clip = splashTheme;
            music.loop = true;
            audioSource.Play();
        }
        else if (levelManager.currentSceneIndex == 1)
        {
            music.clip = mainMenuTheme;
            music.loop = true;
            audioSource.Play();
        }
        else if (levelManager.currentSceneIndex == 2)
        {
            music.clip = levelOneTheme;
            music.loop = true;
            audioSource.Play();
        }
        else if (levelManager.currentSceneIndex == 3)
        {
            music.clip = levelTwoTheme;
            music.loop = true;
            audioSource.Play();
        }
        else if (levelManager.currentSceneIndex == 4)
        {
            music.clip = levelThreeTheme;
            music.loop = true;
            audioSource.Play();
        }
        else if (levelManager.currentSceneIndex == 5)
        {
            music.clip = normalEndingTheme;
            music.loop = false;
            audioSource.Play();
        }
        else if (levelManager.currentSceneIndex == 6)
        {
            music.clip = settingsTheme;
            music.loop = true;
            audioSource.Play();
        }
        else if (levelManager.currentSceneIndex == 7)
        {          
            music.clip = perfectEndingTheme;
            music.loop = false;
            audioSource.Play();
        }
        else if (levelManager.currentSceneIndex == 8)
        {
            music.clip = gameOverTheme;
            music.loop = false;
            audioSource.Play();
        }
        else if (levelManager.currentSceneIndex == 9)
        {
            music.clip = storyBookTheme;
            music.loop = true;
            audioSource.Play();
        }
        else
        {
            ChangeSong();
        }
    }

}