Hymnals are not used at this church for a couple of reasons, although I do not think that Lee actually comes out and says why.
First of all, having hymn books for everyone would cost a lot of money. You can see from the description of the church that they don't have much money. Even the one hymn book the do have is all beat up, for example.
The second reason is that you can assume that most of the people who attend the church do not know how to read anyway. So having books would not help.
So instead of all having books, the music superintendent "lines out" the hymns and they all sing.