All passive mobs (with exceptions) do not have a death sound, they use the hit sound effect

(exceptions are pig, villager, bat, wolf, iron golem, snow golem and horse)

Steps to reproduce

• Spawn any passive mobs except any exceptions (shown above)

• Grab a sword and kill it

• Listen carefully, and compare when hitting it with your hand as to not insta-kill it, the sounds will match
  (sheep don't even have a hit sound, they just play the 'say' sound)

• You can confirm this by searching the the mob's sound folder in the minecraft assets folder