A sound system installed in the walls and/or ceiling may be anything from full range 8" speakers to 15" coaxial ones.

They will do well for background music but are seldom articulate enough for lectures or sermons.

That is best done with one or two medium column speaker systems placed near the podium and adjusted to match the voice of the person speaking.

The installed system also spreads the sound around in all directions which may invite the feedback mentioned.

A separate PA system near the podium can direct the sound foreward and away from the microphone, reducing the chance of feedback.

Also, a good quality mike helps a lot, since the quality of the sound is only as good as the microphone.