I was recently creating a situation in which a player would walk into the room, and a creeper sound would play. I had used the command " /playsound mobs.creeper.say2" the chat would say "[@:Played sound 'mobs.creeper.say2' to myserval] and no sound would play. Though if I did "/playsound mobs.creeper.death" Then it would play the creeper death sound.
I have recorded this here: http://youtu.be/7RO-_BNlL9Q (it's 15 seconds)