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Learning to "project" is a skill. Additionally, when there are no PA systems, people learn where is a good place to speak, and where isn't.
Both the Greeks and Romans built amphitheaters for various purposes, and a hillside can be used as a natural amphitheater. With practice it isn't hard to address over 1,000 people -- the largest group I've ever spoken to without amplification was about 4,000 people.