did some experiments with my friends turns out it is a global sound. might be a bug from last patch cause I dont ever remember it being heard across the map before last patch. I recommend making a bug report about it cause that seems like it should not be a thing. though by proxy it is an interesting method of map control against players who play by sound. a mental is that meant for me or is she still down there? slight hesitation where the player checks their map to be sure.
League is as much mental as it is physical ability to preform, I also know its ridiculous to mention physical performance, but honestly if your fingers are not hitting skills right or your limited in some way that prevents proper execution of skills be it a gamers joint up to missing limbs. also if globals are to have sounds that can be heard across all maps then Ashe, Ezeal, twisted fate, GP, pantheon, Ziggs and any other that I miss that dont make a sound till your either hit or passes right by you should trigger some sort of noise before it touches down. but honestly that would be silly for most of them as TF makes an eye above you a visual warning, Ziggs and pantheon you get the indicator on where it/they are landing, GP's ulti hits as soon as he casts, and ashe and Ez would be silly if they could be heard globally firing their ultis. as for any one else I missed other then Rek and Sion as those are covered in the post above.