I modified your script a bit. If you press volume, the button will be replaced by a slider. If you press resume, the volume will be the value of the slider.

I don't know what the if(waited) does, so I removed it from the update. It just stood there and did nothing.

If you want your slider to be higher or wider, you can add some GUILayoutOptions. Check out the links. I hope this helps :)

http://unity3d.com/support/documentation/ScriptReference/GUILayoutOption.html

http://unity3d.com/support/documentation/ScriptReference/GUILayout.HorizontalSlider.html

public GUISkin myskin;

private Rect windowRect;
private bool paused = false, waited = false;
private bool options;
private float hSliderValue = 0.5f;


private void Start()
{
    windowRect = new Rect(Screen.width / 2 - 100, Screen.height / 2 - 100, 200, 200);
}

private void waiting()
{
    waited = true;
}

private void Update()
{

    if (Input.GetKeyDown(KeyCode.P) || Input.GetKeyDown(KeyCode.Escape))
    {
        if (paused)
            paused = false;
        else
            paused = true;

        waited = false;
        Invoke("waiting", 0.3f);
    }

    if (paused)
    {
        Time.timeScale = 0;
        AudioListener.pause = true;
    }
    else
    {
        Time.timeScale = 1;
        AudioListener.pause = false;
    }
}

private void OnGUI()
{
    if (paused)
    {
        windowRect = GUI.Window(0, windowRect, windowFunc, "Game Paused");
    }
}

private void windowFunc(int id)
{
    if (GUILayout.Button("Resume"))
    {
        paused = false;
        options = false;

        AudioListener.volume = hSliderValue;
    }
    if (!options)
    {
        if (GUILayout.Button("Volume"))
        {
            options = true;
        }
    }
    else
    {
        hSliderValue = GUILayout.HorizontalSlider(hSliderValue, 0.0f, 1.0f);
    }

    if (GUILayout.Button("Return to Main Menu"))
    {
        Application.LoadLevel(0);
    }
}