Ultra simple Music Manager

MusicManager.cs

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

// @kurtdekker - example of an ultra-simple music manager.
//
// How to use:
//	MusicManager.Instance.PlayMusic( Audioclip clip);
//
// Call the above with null to stop music.
//
// Do NOT drop this into a scene!! Just use it from code as above.
//

public class MusicManager : MonoBehaviour
{
	static MusicManager _instance;

	public static MusicManager Instance
	{
		get
		{
			if (!_instance)
			{
				_instance = new GameObject( "MusicManager").AddComponent<MusicManager>();
				DontDestroyOnLoad(_instance);
			}
			return _instance;
		}
	}

	const float crossFadeTime = 1.0f;
	// stays zero when not phasing from one song to the next
	float fading;
	AudioSource Current;
	AudioSource Previous;

	public void PlayMusic( AudioClip clip)
	{
		if (Current)
		{
			if (Current.clip == clip) return;
		}

		if (Previous)
		{
			Destroy( Previous);
			Previous = null;
		}

		Previous = Current;

		Current = gameObject.AddComponent<AudioSource>();
		Current.clip = clip;
		Current.loop = true;
		Current.volume = 0;
		Current.bypassListenerEffects = true;
		Current.Play();

		fading = 0.001f;
	}

	void Update()
	{
		if (fading > 0)
		{
			float fraction = fading / crossFadeTime;

			fraction = Mathf.Clamp01( fraction);

			// crossfade
			if (Previous)
			{
				float volume = 1.0f - fraction;
				Previous.volume = volume;
			}
			if (Current)
			{
				float volume = fraction;
				Current.volume = volume;
			}

			fading += Time.deltaTime;

			if (fraction >= 1)
			{
				fading = 0.0f;
				if (Previous)
				{
					Destroy( Previous);
					Previous = null;
				}
			}
		}
	}
}

TestMusicManager.cs

// example of how to use...

using UnityEngine;

public class TestMusicManager : MonoBehaviour
{
	public AudioClip Song1;
	public AudioClip Song2;

	void Update ()
	{
		if (Input.GetKeyDown( KeyCode.Alpha1))
		{
			MusicManager.Instance.PlayMusic(Song1);
		}
		if (Input.GetKeyDown( KeyCode.Alpha2))
		{
			MusicManager.Instance.PlayMusic(Song2);
		}	
		if (Input.GetKeyDown( KeyCode.Alpha3))
		{
			MusicManager.Instance.PlayMusic(null);	// silence
		}
	}
}