The announcements in The Giver are made over a vast loudspeaker system in order to enforce the rules of the community.
Individuals in this dystopian society are under constant surveillance to make sure that they always toe the line. If they don't, then announcements will be made over the speakers to remind them that they've broken the community's rules.
Announcements also take the form of direct orders that the citizens of this society are expected to obey without question. The omnipresence of the speakers and the never-ending announcements they give out serve to remind everyone that they live in a society without basic freedoms or privacy. They just have to do as they're told.
Even so, as Jonas observes, many people ignore the announcements if they're not directly applicable to them. This is because these announcements apply to selected individuals, who nonetheless are not named. Jonas is on the receiving end of one such message when he takes an apple home in contravention of community rules. The general idea is that those who break the rules will be shamed into compliance.