I mean most town squares have no restriction on using a megaphone, and yet town squares have not been drowned out and rendered useless by megaphones. Even if that did happen, it would be a very poor analogue to generic advertising which can not drown out conversation. At best it would be an argument against megaphones over a certain volume, ie certain methods of communication might be reasonable to restrict, but restricting the ideas that can be expressed by megaphone is indefensible.