In the middle of the game, press pause and type "BANG".

Also, try these sequences:
EFFECT                             SEQUENCE
~~~~~~                             ~~~~~~~~
play as gold dural                 In the character selection menu, press DOWN,
                                  UP, RIGHT, then DEL+LEFT.  You should here a
                                  confirming sound
play as silver dural               In the character select menu, press DOWN, UP,
                                  RIGHT, then DEL+LEFT
add dural to the records screen   At the start screen, press UP 17 times, then
                                  press ENTER.  Go to the options screen, and
                                  you should hear "RING OUT!".
slow motion replaying              At the end of the round, hold DEL+END+PGDN
                                  before the character does their victory
                                  stand
choose victory phrase              Hold either DEL, END or PGDN, and you can
                                  choose
stage select and ring size        At the start screen, press UP 12 times, and
                                  press ENTER.  Go to the options menu, and
                                  you should here "K.O.!".  Move the cursor to
                                  the exit, and press down once more to make
                                  the cursor disappear.  Press DEL and a
                                  second options menu will appear.