System.Windows.Resources.StreamResourceInfo r1 = Application.GetResourceStream(new Uri("1.mp3",UriKind.Relative));
System.IO.StreamReader sr1 = new System.IO.StreamReader(r1.Stream);
System.Windows.Resources.StreamResourceInfo r2 = Application.GetResourceStream(new Uri("2.mp3", UriKind.Relative));
System.IO.StreamReader sr2 = new System.IO.StreamReader(r2.Stream);
System.IO.MemoryStream stream = new System.IO.MemoryStream();
System.IO.StreamWriter writer = new System.IO.StreamWriter(stream);
writer.Write(sr1.ReadToEnd());
writer.Write(sr2.ReadToEnd());
MediaElement dingdong = new MediaElement();
dingdong.SetSource(stream);
LayoutRoot.Children.Add(dingdong);
dingdong.Play();