We do not really know who exactly is writing the announcements (we do know that there is someone called the "Speaker" who sits in an office all day making announcements) or how they are transmitted everywhere in the community. However, what we do know is that every home and every office in the community has a speaker so they can hear the announcements. Only the Receiver (so far as we know) is allowed to turn off the speaker so as not to be disturbed.
It is also not clear that everyone hears the same announcements. It seems silly that everyone in the community would hear an announcement, for example, telling 11 year-old males what to do.