Downloaded mp3 stream is not Playing from IsolatedStorage in windows phone 7

Question

I have downloaded a .mp3 Song in my application using HttpWebRequest and WebClient Both. I am saving the response stream in Isolated Storage with .mp3 extension filename. When i m trying to play it with BackgroundAudioPlayer it do not plays.

Here is my code. Please suggest what is wrong with it.

using (IsolatedStorageFile store = IsolatedStorageFile.GetUserStoreForApplication())
{
       if (store.FileExists(lstBoxSongList.SelectedItem as string))
       {
             using (IsolatedStorageFileStream isfStream = store.OpenFile(lstBoxSongList.SelectedItem.ToString(), FileMode.Open, FileAccess.Read))
             {
                    AudioTrack track = new AudioTrack(new Uri(lstBoxSongList.SelectedItem as string, UriKind.RelativeOrAbsolute), lstBoxSongList.SelectedItem.ToString(), "", "", null);
                    BackgroundAudioPlayer.Instance.Track = track;
                    BackgroundAudioPlayer.Instance.Volume = 1;
                    BackgroundAudioPlayer.Instance.Play();
             }
       }
}

The way i m saving the Downloaded stream into IsolatedStorage is given below.

webClient.OpenReadCompleted += (s1, e1) =>
            {
                if (e1.Error == null)
                {                     
                        bool isSpaceAvailable = IsSpaceIsAvailable(e1.Result.Length);

                        if (isSpaceAvailable)
                        {
                            using (IsolatedStorageFile store = IsolatedStorageFile.GetUserStoreForApplication())
                            {

                                // Save mp3 to Isolated Storage
                                if (store.FileExists(filename))
                                {
                                    store.DeleteFile(filename);
                                }
                                using (var isfs = new IsolatedStorageFileStream(filename, FileMode.CreateNew, IsolatedStorageFile.GetUserStoreForApplication()))
                                {
                                    long fileLen = e1.Result.Length;
                                    byte[] b = new byte[fileLen];
                                    e1.Result.Read(b, 0, b.Length);
                                    isfs.Write(b, 0, b.Length);
                                    isfs.Flush();
                                }
                            }
                       }
                }
           }   

Please suggest what is wrong with this code.

Here is the Xaml code for ListBox containing my file names.

<ListBox Name="lstBoxSongList" SelectionChanged="lstBoxSongList_SelectionChanged" >
                        <ListBox.ItemTemplate>
                            <DataTemplate>
                                <StackPanel>
                                    <TextBlock Text="{Binding}"/>
                                </StackPanel>
                            </DataTemplate>
                        </ListBox.ItemTemplate>
                    </ListBox>