docs: function(id) {
        var self = this;
  
        // Check the passed sound ID (if any).
        if (typeof id === 'number') {
          var sound = self._soundById(id);
          return sound ? !sound._paused : false;
        }
  
        // Otherwise, loop through all sounds and check if any are playing.
        for (var i=0; i<self._sounds.length; i++) {
          if (!self._sounds[i]._paused) {
            return true;
          }
        }
  
        return false;
      }