Well... addons can remove the /yells. I don't know offhand how to remove the actual sound effects. If you can find the appropriate .wav filename, you could override it with an empty one. [edit: without using an addon, that is.]
Does Blizzard consider overriding a wav file to be "tampering"? would the client complain? While your motives are just, I am afraid you may run afoul of some of the same protections/restrictions against swapping out in game textures (naked hacks and the like)
I know FishingBuddy turns up environment sounds while turning others down, but I think that's just twiddling the existing volume knobs. My guess is that Wow will detect modificaiton of core files either shut down or crash.
You can override sounds by [re-]creating the sound file's path and putting your own .wav file there, similar to how some addons in the 1.x days would replace FrameXML files. This is not modifying the MPQ files at all.
The only sound change I've done is to create 'Sound/Spells/Screech.wav' as a zero-length file, because I play with "play sound at character" enabled. And when I'm tanking and there's one of those goddamn bats right next to my toon, the nonstop demoralizing-roar-esque squealing drives me fucking insane inside of five seconds. (The hunters controlling the bat never care; they're standing way back and can't hear the noise.)
This has been described on the UI forum multiple times, and has never been called out by a blue. If they want to disallow it, they only have to say so; until and unless that happens I've concluded it's permitted. (Also, paper tiger scare tactics would have to annoy me a lot more than the fucking bat screeching to get me to stop.)