Apparently this is one of those bugs that isnt a bug.....
ISSUE: mk2 When you insert a coin a sound is played. In Mortal Kombat 2, the sound is a voice that says EXCELLENT, but when you insert a coin during the attract mode the sound doesn't play. It only fails when you insert a coin during the attract mode, but it works when a coin is inserted when you are at the character select screen or playing.
NOTES: "OK, tested on the real PCB. This is correct behaviour. If you insert during attract mode, there is no sound, but if you insert during play/character select, there is a sound." - Reznor007