once you have defeated Harley Quinn and have jumped down the hole there is a room to the left. does anyone know how to get in? also when you go down the next set of steps there is a door to be opened under them but the handle is on the other side?
To the right of the room with all the musical instruments, use Superman's heat vision to blast the padlock, destroy everything, you'll see a blue "magnetic" object (bicycle, IIRC). Use Robin's Magnetic Suit or Cyborg, then you can drop down. Go right to open that hidden door (with Super Strength), or to the left for the hidden area.