I've had this problem, actually have lost alot from it aswell. I was going to post this, but decided to dig around, and found this. I'm not sure what's wrong with it, but only half the mobs are attackable. Very frustrating indeed, sir.
Sounds like a sound error when a player is close to said mob. Is it possible, that when you died you didn't hear it, but it did play? if not, sounds like a coding error with the game itself, causing it NOT to play at times.
Originally placed in Survival, I was helping my friend rebuild, and decided to report this real quick. Also, will add the link for the other anvil falling through the torch, and deleting it in just a minute.