public void mute(String playerId) {
   SqueezePlayer player = getPlayer(playerId);
   if (player == null) return;
   if (player.getVolume() > 0) {
     setVolume(playerId, 0);
   }
 }
 public void unMute(String playerId) {
   SqueezePlayer player = getPlayer(playerId);
   if (player == null) return;
   if (player.getVolume() == 0) {
     setVolume(playerId, player.getUnmuteVolume());
   }
 }
 public void volumeDown(String playerId) {
   SqueezePlayer player = getPlayer(playerId);
   if (player == null) return;
   setVolume(playerId, player.getVolume() - VOLUME_CHANGE_SIZE);
 }