Batman Returns (Game Gear)
This game has a hidden sound test.
Hold Start before turning the Game Gear on. This screen will appear after the Sega logo fades away. You can do this with a cart from any region, but it will only work in a Japanese console (or an emulator set to Japanese mode). Game Genie code 181-05B-6EA can be used to access the sound test on a Game Gear from any region (or an emulator set to any region).