These two first mentioned abilities have no Audio cue atm although they both are either an ult or deal more damage than many ults, also both had quite a distinct sound cue in the past.

Ballista sounds like an old MG42 mowing down bones.
Scourge had an equal sound to assassin's will which has the loudest Audio cue in the game for any ability.

Ballista also managed to get rid of all visual effect on the target, back when it worked right it looked like yellowish golden baloons popped right below your characters ass, also the "bow" was visible, the latter being only visible from time to time and not visible for everyone ( friend of mine got hit by ballista and didn't had any visuals on his screen while i saw it and vic versa

Hearing or seeing the visual and Audio cues for meteor got gambling with Dragon Bones and hasn't been fixed up to this day.
In my 3 man group we have never had one meteor targeted at us that we all could see or hear, with no one noticing it due to abstinence of sound and visual effects being the majority of results in 4 months of pvp.
I won't even talk about the meteor rubberband and meteors ignoring block even after the damage taken from it.