Mike Stevenson's modified MusicManager from Gravity Ghost

MusicManager.cs

using UnityEngine;
using System.Collections;

[System.Serializable]
public class MusicTrack
{
  public string name = "";
	public AudioClip clip;
	public AudioClip clipSecondary;
}

public class MusicManager : MonoBehaviour
{
	public MusicTrack[] tracks;
	public float pauseBetweenTracks;

	ShuffleBag<MusicTrack> randomTracks;

	private float _crossFade;
	public float CrossFade {
		get {
			return _crossFade;
		}
		set {
			source.volume = Mathf.Clamp01 (1 - value);
			source2.volume = Mathf.Clamp01 (value);
			_crossFade = value;
		}
	}

	AudioSource source;
	AudioSource source2;

	bool MutedInEditor {
		get {
			return PlayerPrefs.GetInt ("MuteMusic", 0) == 1;
		}
	}

	void Start ()
	{
		if (Application.isEditor && MutedInEditor) {
			return;
		}

		GameObject music = new GameObject ("_Music");
		music.transform.parent = transform;
		DontDestroyOnLoad (music);
		source = music.AddComponent<AudioSource> ();
		source2 = music.AddComponent<AudioSource> ();
		CrossFade = 0;
		randomTracks = new ShuffleBag<MusicTrack> (tracks);
		randomTracks.Next ();
		StartCoroutine ("PlayTracks");
	}

	public void Play (MusicTrack track)
	{
		if (Application.isEditor && MutedInEditor)
			return;

		source.Stop ();
		source2.Stop ();
		source.clip = track.clip;
		source2.clip = track.clipSecondary;
		source.Play ();
		source2.Play ();
	}

	IEnumerator PlayTracks ()
	{
		while (true) {
			while (source.isPlaying) {
				yield return null;
			}
			yield return new WaitForSeconds (pauseBetweenTracks);
			Play (randomTracks.Next ());
		}
	}
}