Throughout the roman world, there are amphitheaters which could hold 20,000 people or more. I have personally been to ones in Ephesus (where Paul addressed a large crowd) Pergamum, and Pompeii.
Note that both the Sermon in the Mount and the Sermon on the Hill were delivered at landforms that formed natural amphitheaters. If Jesus was standing at the base, rather than at the top, the acoustics would have worked in his favor.
As a speaker who has learned to project, I can tell you that it isn't difficult to do- it is just that amplification helps make it louder. Amplification lets people spread out more and helps when people in the crowd are talking, but large crowds could hear Greek plays without it.
The point here is simple- a speaker has better range than you might imagine. The speaker would need to be loud, for sure, but people throughout the ages have addressed large crowds.
Finally, there is always the monty Python answer and it applies to any sort of dairy producer, really.