code: function(key) {
      var self = this;
      var sprite = self._spriteMap[key];
  
      // Play the sprite sound and capture the ID.
      var id = self.sound.play(sprite);
  
      // Create a progress element and begin visually tracking it.
      var elm = document.createElement('div');
      elm.className = 'progress';
      elm.id = id;
      elm.dataset.sprite = sprite;
      window[key].appendChild(elm);
      self.sounds.push(elm);
  
      // When this sound is finished, remove the progress element.
      self.sound.once('end', function() {
        var index = self.sounds.indexOf(elm);
        if (index >= 0) {
          self.sounds.splice(index, 1);
          window[key].removeChild(elm);
        }
      }, id);
    }
docs: Play a sprite when clicked and track the progress.
  @param  {String} key Key in the sprite map object.