var videoPath = 'video.mp4';
var writestream = fs.createWriteStream('song.mp3');
var ffmpegProc = new ffmpeg({source:videoPath, timeout: 20})
          .duration(900) //15 min
          .format('mp3')
          .audioBitrate(128)
          .on('end', function() {
            fs.exists(videoPath, function(exists) {
              if(exists) {
               fs.unlink(videoPath, function(err) {
//Do smth. with err
				});
              }
            });
          })
          .pipe(writeStream, {end: true});