title: Extracting of MetaData with MediaMetadataRetriever has unexpected result
body: <p>I'm trying to build my own music player for Android.</p>
  
  <p>For a listing I need the metadata of the songs, so I'm using the <code>MediaMetadataRetriever</code> to extract them, but the retriever is unable to extract the metadata for most of the songs (~630 of 669). In my PC music software, the metadata are visible and in the Windows Explorer as well. If I open the properties of the songs, I can modify them and if I load the changed files on the phone, it still doesn't work. By the way: If I use any other player on the phone, it is always able to load the metadata.</p>
  
  <p>Is the <code>MediaMetadataRetriever</code> buggy and should I use some other libraries for extracting or am I doing something wrong?</p>
  
  <p><a href="https://stackoverflow.com/questions/12465915/android-mediametadataretriever-returns-null-values-from-most-keys">This</a> question is related, but doesn't really help to solve my problem.</p>
  
  <p>Here is some of my code as an example:</p>
  
  <pre><code>MediaMetadataRetriever lMetaData = new MediaMetadataRetriever();
  lMetaData.setDataSource(mAbsolutePath);
  mArtist = lMetaData.extractMetadata(MediaMetadataRetriever.METADATA_KEY_ARTIST);
  mTitle = lMetaData.extractMetadata(MediaMetadataRetriever.METADATA_KEY_TITLE);
  mAlbum = lMetaData.extractMetadata(MediaMetadataRetriever.METADATA_KEY_ALBUM);
  mAlbumArtist = lMetaData.extractMetadata(MediaMetadataRetriever.METADATA_KEY_ALBUMARTIST);
  mGenre = lMetaData.extractMetadata(MediaMetadataRetriever.METADATA_KEY_GENRE);
  mSongNumber = lMetaData.extractMetadata(MediaMetadataRetriever.METADATA_KEY_CD_TRACK_NUMBER);
  mYear = lMetaData.extractMetadata(MediaMetadataRetriever.METADATA_KEY_YEAR);
  
  if(mArtist == null)
      mArtist = new String("");
  if(mTitle == null) 
      mTitle = new String("");
  if(mAlbum == null) 
      mAlbum = new String("");
  if(mAlbumArtist == null) 
      mAlbumArtist = new String("");
  if(mGenre == null) 
      mGenre = new String("");
  if(mSongNumber == null) 
      mSongNumber = new String("");
  if(mYear == null) 
      mYear = new String("");
  </code></pre>