I seriously considered replacing the whistle with the fel reaver noise at one point. Then again, I replaced the bear aggro noise with the fel reaver sound at one point (long live fel bear!) and replaced the darkmoon carousel song with windmill hut/song of storms at one point, so I'm not sure I'm the best or most steadfast sound lady on the planet and my personal sound choices should not be considered wise or canon lol :x
Replacing/muting sound files is actually reasonably harmless, since you're never actually deleting or replacing any game files and it's very similar to installing an addon, only with a replaced/quiet sound file instead of an addon file. I think about the worst you could hurt the game files by doing so is accidentally changing/muting the wrong one (which is fixed by removing the file you just added) or not placing/naming the file properly (which only adds the size of your misplaced file to your wow folder and doesn't affect the game's running at all).
The hardest part is finding out the name and file path of the sound file you want to replace; after that it's just creating the correctly-named folders and files in your WoW folder. Like, this is my worgen sniff mute path (which has been in place since 2010 apparently; do worgens even still sniff? haha):
You can do the same with UI files also (that's what those extra folders in my Interface folder are for; I've replaced the Overwatch icons from battle.net with the generic battle.net icons because Overwatch makes me unreasonably angry for Mac Reasons™); way way waaaay back in the day, you could do the same with models and textures as well, but that was patched out pre-TBC and was also a lot dodgier and not technically allowable anyway (and then you would replace the actual game files, which was REALLY dodgy and actually risky from a corruption and banning standpoint but some people did/continue to do anyway)
I can totally understand not wanting to mess with it, though—it took me worgen sniffing and sabercat snorgling (both of which hit the wrong part of my ear and make me deeply uncomfortable) to actually bite the bullet and learn how to do it for myself, but it's been really nice to know how to do for a variety of reasons.