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. Additionally, it would probably prudent to add a "forced" tag which is false on all naturally occurring sounds (except death sounds) but true on sounds created with /playsound or /execute /playsound, and will prevent these sounds from being stopped upon the death of their source. It's a lot of extra coding for a relatively small change, but SSP will feel much more polished with it implemented IMO.