docs: function() {
      var self = this;
  
      // Loop through all active sounds and update their progress bar.
      for (var i=0; i<self.sounds.length; i++) {
        var id = parseInt(self.sounds[i].id, 10);
        var offset = self._sprite[self.sounds[i].dataset.sprite][0];
        var seek = (self.sound.seek(id) || 0) - (offset / 1000);
        self.sounds[i].style.width = (((seek / self.sound.duration(id)) * 100) || 0) + '%';
      }
  
      requestAnimationFrame(self.step.bind(self));
    }