private static void GroupChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var oldGroup = e.OldValue as SoundPlayButtonGroup;
var newGroup = e.NewValue as SoundPlayButtonGroup;
if (oldGroup != null)
oldGroup.UnRegister(d as SoundPlayButton);
if (newGroup != null)
newGroup.Register(d as SoundPlayButton);
}