1 SingletonBehaviour.cs

public abstract class SingletonBehaviour<Interface> : MonoBehaviour
    where Interface : class
{
    private static Interface _instance = null;
    public static Interface instance { get { return _instance; } }

    void Awake()
    {
        System.Diagnostics.Debug.Assert(this is Interface);

        if (_instance == null)
        {
            _instance = this as Interface;
            DontDestroyOnLoad(gameObject);
            OnAwake();
        }
        else
        {
            DestroyImmediate(this);
        }
    }

    protected virtual void OnAwake()
    {
    }
}

2 AudioManager.cs

// define interface for the singleton (simplifies intellisense and access
// when writing scripts that use the singletons since they won't see all
// the standard Unity behaviour methods and properties)
public interface IAudioManager
{
    bool audioEnabled { get; set; }
    float musicPlayTime { get; }
    void PlaySfx(string name);
    void ToggleAudio();
}

// subclass the SingletonBehaviour using the interface as the type parameter.
public class AudioManager : SingletonBehaviour<IAudioManager>, IAudioManager
{
    private const string audioEnabledKey = "IsAudioEnabled";

    [SerializeField]
    private AudioSource _musicSource = null;

    [SerializeField]
    private AudioSource _sfxSource = null;

    [SerializeField]
    private AudioClip[] _soundEffects = null;

    public bool audioEnabled
    {
        get { return !_musicSource.mute; }
        set
        {
            if (_musicSource.mute == value)
            {
                _musicSource.mute = !value;
                _sfxSource.mute = !value;
                PlayerPrefs.SetInt(audioEnabledKey, value ? 1 : 0);

                if (!value)
                {
                    _musicSource.Stop();
                }
                else
                {
                    _musicSource.Play();
                }
            }
        }
    }

    public float musicPlayTime
    {
        get { return _musicSource.time; }
    }

    protected override void OnAwake()
    {
        audioEnabled = !PlayerPrefs.HasKey(audioEnabledKey) || PlayerPrefs.GetInt(audioEnabledKey) == 1;
    }

    public void PlaySfx(string name)
    {
        foreach (var clip in _soundEffects)
        {
            if (clip.name == name)
            {
                _sfxSource.PlayOneShot(clip);
                break;
            }
        }
    }

    public void ToggleAudio()
    {
        audioEnabled = !audioEnabled;
    }
}

3 ExampleUsage.cs

// from somewhere else in your game you now just access the interface
// through the type and .instance property
AudioManager.instance.PlaySfx("MenuSelect");