Even with the addition of the tower, all of the gathered people would not be able to hear, therefore Benjamin causes the speech to be written. Again there is more information in this verse than would first appear.
One of the interesting questions we might ask is why a large group of people would hang around if they could neither hear, nor probably see, the person they came to hear and see. The answer is that this occasion is more that the speech. For many of them, the celebration and the chance to visit with friends (not to mention the feasting) would be reason enough to come and stay. The audience would have had multiple reasons for coming to the speech. Many of them were fulfilled even when they could not hear nor see.
The second piece of information is that the speech was written down expressly so that the people could have his words. We know that Nephite society was literate, but we do not know the extent of the literacy. It would be a very unusual society in the ancient world where there was a very high literacy rate, and particularly in Mesoamerica among those who were farmers in the fields. While the writing of the speech might mean that it was distributed so that many could read it, this would also require a large number of copies and a large amount of material on which to write, in addition to a significant amount of time.
Given the probable illiteracy of the general populace, the difficulty in mass producing writing, and the fact that the material on which the speech would be written had to have been manufactured locally or traded for, it is more likely that there were a limited number of copies created so that emissaries from the king could read the speech to the gathered people in the hamlets.