code: 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;
      }
docs: Check if a specific sound is currently playing or not (if id is provided), or check if at least one of the sounds in the group is playing or not.
  @param  {Number}  id The sound id to check. If none is passed, the whole sound group is checked.
  @return {Boolean} True if playing and false if not.