Well I don't really feel like modifying someone else script to produce a minor feature.
So this should do it for you.
CODE
# Play SE on Critical Hit
# Code Snippet by DiamondandPlatinum3
class Game_Battler
#==================================================
# EDITABLE REGION
#==================================================
CRITICAL_HIT_SE = "Applause"
CRITICAL_HIT_VOLUME = 80
CRITICAL_HIT_PITCH = 100
#==================================================
alias playsound_oncritical make_attack_damage_value
def make_attack_damage_value(*args)
playsound_oncritical(*args)
RPG::SE.new(CRITICAL_HIT_SE, CRITICAL_HIT_VOLUME, CRITICAL_HIT_PITCH).play if @critical
end
end