为什么，打开的 中没有MP3文件可以添加，“我的音乐”里面确实是有MP3文件的？？！！

 private void openBtn_Click(object sender, RoutedEventArgs e)
        {
            ShellContainer selectedFolder = null;
            selectedFolder = KnownFolders.Music as ShellContainer;
            CommonOpenFileDialog cfd = new CommonOpenFileDialog();
            cfd.InitialDirectoryShellContainer = selectedFolder;
            cfd.EnsureReadOnly = false;
            cfd.Filters.Add(new CommonFileDialogFilter("WMV Files", "*.wmv"));
            cfd.Filters.Add(new CommonFileDialogFilter("AVI Files", "*.AVI"));
            cfd.Filters.Add(new CommonFileDialogFilter("MP3 Files", "*.MP3"));

            if (cfd.ShowDialog() == CommonFileDialogResult.OK)
            {
                mediaElement.Source = new Uri(cfd.FileName, UriKind.Relative);
                playBtn.IsEnabled = true;
            }
        }

犯了一个不该犯的错误，不说是什么了，代码也没错，什么都没错，哈哈哈！是无意间发现的