Gangplank, Kalista, and Rek'sai all have global VO when they ult
But
still doesn't.
Is it because Riot doesn't like "audio clutter"? Well then turn off Kalista's global VO, I don't give a fuck about her stupid oathsworn bond thing. And do we really need to hear GP laugh when he ults? I mean, the ult is right on top of you, it's not like you need an indicator saying "hey, GP just ulted right here, better watch out!"
Rek'sai at least kinda makes sense, but Karthus's ult has such a long cooldown that any clutter it adds would be minimal IMO.