This bug has been around for so long I've just accepted it as a fact of Minecraft life, but since Mojang is trying to polish survival gameplay in 1.9, I think they should take a look at this. Basically, when a mob is killed, any sounds it was making previously keep playing, which is a bit disorienting in the cases of Creepers and Endermen. Most of the time I'm not able to kill a creeper in melee without triggering its hiss sound when it dies, and since the sound is centered at a position in the world, if I run past the dying Creeper as it hisses it sounds like there's another one behind me about to explode. As for Endermen, their aggro sound is really long-lasting, and if you kill one in just a few hits it sounds like they're still coming for you. What I'd like to see is for all of a mob's sounds except for its death sound to immediately stop playing if the mob is killed.

I imagine fixing this issue will involve giving all sound events some sort of "source" tag to track the entity (if any) which played the sound originally. Ideally /playsound would also be updated to accept an additional, optional boolean (false by default) for deciding whether to tie the sound to the executor.