I've been having this too. It's random, not every game. The Announcer works for the opening announcement (Welcome, 30 Seconds, MInions have Spawned) but after that no notice on kills, towers/inhibs being destroyed, etc. I never notice until halfway through the game though, when I personally destroy a tower and get no notification of it.
I noticed it first in Ranked when I said that we hadn't gotten a single tower down, and my teammate politely pinged the two towers bot lane that had been destroyed without a single announcement.