Overall speaking, using a loud hailer speaker or megaphone in public places is not against the law on its own. However, it mainly depends on where you are and why you are using the megaphone. Also, some parts of the world do not allow public use of these devices. While at other places, it depends on how loud you are keeping the volume of the megaphone while addressing. But using them for music and personal fun and entertainment activities in personal automobiles, the use is strictly prohibited in most parts of the world.