'Func-def not allowed...' error on IR MP3 controller

You probably meant:

void loop() {
  int key = getIRKey();  
  if (key !=0) {
    switch(key) {
      case 144: MP3player.playTrack(1); break;
      case 145: MP3player.playTrack(2); break;
      case 146: MP3player.playTrack(3); break;
      case 147: MP3player.playTrack(4); break;
    }
  }
}