code: public PayloadBuilder sound(final String sound) {
          if (sound != null) {
              aps.put("sound", sound);
          } else {
              aps.remove("sound");
          }
          return this;
      }
docs: Sets the alert sound to be played.
  
  Passing {@code null} disables the notification sound.
  
  @param sound the file name or song name to be played
  when receiving the notification
  @return  this