[CustomEditor(typeof(AudioManager))]
public class AudioManagerEditor : Editor
{
    float volume = 1f;

    public override void OnInspectorGUI()
    {
        DrawDefaultInspector();

        volume = EditorGUILayout.Slider("Volume", volume, 0f, 1f);

        if (GUILayout.Button("Play Sound"))
        {
            AudioManagert = (AudioManager)target;
            if(t.clip != null)
                AudioSource.PlayClip(t.clip, volume);
        }
    }
}