I believe this started with Patch 5.22 for the preseason. The announcer no longer gives audio for every single kill like it used to. It still calls multi-kills, and an unknown pattern for other individual kills. Is there a setting or something to re-enable this? I like the audio cue to know how many champions are still alive, particularly in large teamfights where it can be hard to keep track of which particles mean something is dead or not.
According to my researches (which came up empty) i would assume that somewhere in the game code they must've made so the sounds don't overlap (so you won't hear double and triple kill at the same time) and i guess (again sorry for not being sure of anything, these are assumptions and speculations, but i couldn't find anything online) that if a certain time passes they no longer play the audio since it's pointless by then.
Again, do not take anything i said has a certainty this is all speculation on my part , for which i am posting to try and give some possible reasons has to why this happens (again, not sure on anything!!)
This question is something either a Riot Staff or a person who know's or has access to the actual game code (Or someone who knows someone there, you get where i'm headed don't you?) to be able to specify the reason, it might even be an actual bug in the game that Riot themselves haven't noticed, or have but haven't made any impactfull changes to fix it since it doesn't affect the actual gameplasy (What's wrong with missing the announcemment on a kill or two? You can still see the kill feed and score).
What i'm trying to say is, all we can do is wait until Riot either fix this, or just cry about the announcer not saying Pentakill when you are playing Full Ap Amumu (It works some games!)