This secret menu is available in any of the games in Williams Arcade's Greatest Hits. Just pick a game and enter it. Once in the game, hold down L1 and L2 buttons and press Select and Start simultaneously. First, a game status screen will come up. Press X to skip to the options screen, where you can adjust the difficulty, number of lives, and other settings of the game. Once you're finished, hit Start to return to gameplay.
Mortal Kombat Sound Test
Turn on the PlayStation without a CD inside and access the CD player from the menu. Now insert the game CD and 2 tracks will be indicated on the screen. Choose to play track 2 and you will hear over 5 minutes of Mortal Kombat 3 sounds.