Good news! The MAME FAQ has your answer.
http://mamedev.org/devwiki/index.php/FAQ:Games#The_volume_in_the_Mortal_Kombat_and_other_recent_Williams_games_is_very_low.
The volume in the Mortal Kombat and other recent Williams games is very low.
When the game is running, hit F2 to get to its own internal service menu.
Hit F2 again to "close" the service switch so that you won't get stuck in the menu.
Now navigate to the "Volume adjust" option with arrow keys or your joystick, and adjust the volume.
Then exit the menu with the appropriate selections.