Why is the Chapter 1 entitled "The Sound of the Shell?"
I would say that it is named this because the shell (the conch) stands for the most important theme of this chapter.
What's happening in this chapter sets up the tension in the whole book, and that tension revolves around the shell and what it stands for. Ralph uses the shell to call people to the meeting. At the meeting, he is going to try to organize them -- to make them into a civilization. To do that, he needs them to obey rule and one of the rules is that you come when you hear the conch and you listen to the person holding it.
But that is in tension with what Jack wants. He wants a wilder society -- one where there aren't rules and the strongest rule. The rest of the book will largely be about this -- will the sound of the shell command respect, or will it not?
The shell in chapter I represents civilization and order. When Ralph and Piggy first meet they are not as aware of the seriousness of the situation. Ralph is being playful and flying around like an airplane. They come across the shell which they work together at pulling up. The shell at first is represented by Piggy as something expensive (a remnant of commercialism and the other social standards).
Piggy suddenly realizes that the shell can be used to call together the other boys. Ralph blows the conch and the other children come to them.
"Signs of life became visible now."(18)
Jack shows up with some of his choir boys. He already demonstrates a look of anger. He tries to claim the title of chief but is voted out in favor of Ralph. It is the first of the two boy's conflicts.
The conch is used as a mediating stick which is passed around. He that holds it has the right to speak. It is used to call order to the situation and establish the rules of civilization. In many ways it is the last of the rules.