MusicPlayer.cs

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

public class MusicPlayer : MonoBehaviour
{
    static MusicPlayer _instance;

    public static AudioSource ActiveSource
    {
        get
        {
            return _instance._sources[_instance._activeSourceIndex];
        }
    }
    
    public static AudioSource IdleSource
    {
        get
        {
            int idleIndex = _instance._activeSourceIndex == 0 ? 1 : 0;
            return _instance._sources[idleIndex];
        }
    }

    public AnimationCurve FadeCurve;

    AudioSource[] _sources;
    int _activeSourceIndex;

    void Awake ()
    {
        _instance = this;
        _sources = GetComponents<AudioSource>();
        _activeSourceIndex = 0;
    }
    
    public static void Play (string trackName)
    {
        AudioClip track = Resources.Load<AudioClip>(trackName);
        Play(track);
    }
    
    public static void Play (AudioClip track)
    {
        ActiveSource.loop = true;
        ActiveSource.clip = track;
        ActiveSource.volume = 1;
        ActiveSource.Play();
    }
    
    public static void PlayOneShot (string trackName)
    {
        AudioClip track = Resources.Load<AudioClip>(trackName);
        PlayOneShot(track);
    }
    
    public static void PlayOneShot (AudioClip track)
    {
        ActiveSource.loop = false;
        ActiveSource.Play(track);
    }
    
    public static void Stop ()
    {
        ActiveSource.Stop();
    }
    
    public static void CrossFade (string trackName, float fadeTime = 1.0f)
    {
        AudioClip track = Resources.Load<AudioClip>(trackName);
        CrossFade(track, fadeTime);
    }
    
    public static void CrossFade (AudioClip track, float fadeTime = 1.0f)
    {
        _instance.StartCoroutine(_instance.CrossFadeAsync(track, fadeTime));
    }
    
    public static void FadeOut (float fadeTime = 1.0f)
    {
        _instance.StartCoroutine(_instance.CrossFadeAsync(null, fadeTime));
    }
    
    void ChangeSource ()
    {
        _activeSourceIndex = _activeSourceIndex == 0 ? 1 : 0;
    }
    
    IEnumerator CrossFadeAsync (AudioClip incomingTrack, float fadeTime)
    {
        IdleSource.clip = incomingTrack;
        IdleSource.volume = 0;
        
        if (IdleSource.clip != null)
            IdleSource.Play();
    
        for (float t = 0; t < fadeTime; t += Time.deltaTime)
        {
            float s = t/fadeTime;   
            ActiveSource.volume = FadeCurve.Evaluate(1 - s);
            IdleSource.volume = FadeCurve.Evaluate(s);
            yield return null;
        }
        
        ChangeSource();
        ActiveSource.loop = true;
        IdleSource.Stop();
        IdleSource.clip = null;
    }
}