var mediaItemArray = Cc["@songbirdnest.com/moz/xpcom/threadsafe-array;1"].createInstance(Ci.nsIMutableArray);

    mediaItemArray.appendElement(mediaItem, false);

   if (mediaItemArray.length > 0) {
       var metadataService = Cc["@songbirdnest.com/Songbird/FileMetadataService;1"].getService(Ci.sbIFileMetadataService);

       // This will write out the properties in propArray for each item.
       var propArray = ArrayConverter.stringEnumerator(SBProperties.lyrics);
       var job = metadataService.write(mediaItemArray, propArray);
       }