string url = re[\"response\"][0][\"url\"].ToString();
MediaElement mm = new MediaElement();
mm.Source = new Uri(url,UriKind.RelativeOrAbsolute);
mm.AutoPlay = tr
You need to add your MediaElement to your VisualTree before playing it, since you're creating it in the codebehind. For example, assuming you have LayoutRoot and that your url is correct, this should work.
string url = re["response"][0]["url"].ToString();
MediaElement mm = new MediaElement();
mm.Source = new Uri(url,UriKind.RelativeOrAbsolute);
mm.AutoPlay = true;
mm.Volume = 0.7;
LayoutRoot.Children.Add(mm);
mm.Play();