Ok, I am not sure if I can fully answer this question, but I'll try since noone else seems to be able to say something about this. I tried this stuff in custom games, so I am pretty sure this is correct:
- There is no "hearing range". Obviously you do hear some actions, but everything you hear is limited to your camera vision, which obviously means you have to have vision over the area and you have to watch it if you want to hear anything.
- You should not be able to hear the abilities used around the corner or over a wall (like in the scenario you described at blue buff), but sometimes, you are able to see some abilites to the fog of war (just like Shaco's Decieve), but most of these vision are bugs. Examples for such abilities are Malphites Ground Slam, Maokais Arcane Smash or Lee Sins Sonic Wave. Since these abilities are only visible due to bugs, you can't or at least you shouldn't really count on beeing able to see them.
- You cannot hear skills used from bushes unless its warded or you have vision through other skills/spells, but if an ability goes out of a bush (like Leonas E or Nidalees Spear) and you're watching that bush, you can hear the spell as soon as he leaves the bush, even if you dont have vision in the bush itself, or if the spell misses.
So I'd say seeing = hearing. If you are able to catch an area (Thats not in fog of war) with your camera, you can hear everything you have vision off.
Something important to note: If you use a skill into a bush or an area you have no vision of (Fog of War), and this skill makes a certain sound if it hits something, you will hear that sound if you hit someone, even tho you have no vision over the target itself. Premise for this is obv you "watching" the area.