Back on the topic, Jesus also addressed a large crowd from a boat one time as well, although we don't know how large. In the movie The Gospel of John (the best gospel adaptation I've ever seen BTW) they fascinatingly portray Jesus' preaching style as dynamic and interactive, walking among the people as he talked. He very well may have used both stationary and itinerant methods of delivering his sermons, whichever was best suited to addressing the crowd at hand in a given situation.
And plus, as I pointed out George Whitefield preached to five (or more) times the number that were present when Jesus multiplied the bread, so it's not at all inconceivable that if he did preach to them that they easily could have heard him from the mountain side.