In "The Loudest Voice" by Grace Paley, in the the last sentence, how does Shirley's voice being the loudest make her think her prayer will be heard?

In Grace Paley's short story The Loudest Voice, young Shirley Abramowitz's particularly loud voice is, in a sense, her salvation. In the noisy, vociferous world in she exists, it is her natural ability to make herself heard above the fray that sets her apart. A point of contention with her mother and with the grocer, who complains at one point that Shirley's voice is causing the labels to peel off of the cans of soup he stocks in his store, the young girl's father views his daughters ability to project more encouragingly, noting that, "in the grave it will be quiet." Indeed, it is Shirley's voice that attracts the attention of Mr. Hilton, the sixth grade teacher who enlists her for the annual school Christmas production. Sure enough, Shirley's ability to be heard proves an asset, as her role in the production assumes greater importance than merely acting as narrator: "Every day was busy and full of experience. I was Right-hand Man. Mr. Hilton said: 'How could I get along without you, Shirley!'"
Paley, her story told retrospectively, emphasizes the special qualities of her protagonist. Shirley is imbued not only with a high-octane voice, but with the confidence to navigate her way through life. It is for this reason that she prays with the confidence of one who knows she will be heard. Her voice has enabled her to rise above her classmates and to be heard within the audible clamor of her community. That is why Paley concludes her brief story with Shirley commenting on her act of prayer:

"I was happy. I fell asleep at once. I had prayed for everybody: my talking family, cousins far away, passersby, and all the lonesome Christians. I expected to be heard. My voice was certainly the loudest."

If Shirley's voice enables her to be heard among those in whose midst she travels, then it stands to reason, in her young mind, that that same attribute makes possible her superior ability to heard by God.