Soundtrack by Chloë Grace Moretz

Since you say that English is not your first language -- your English is not bad, BTW -- you may not be familiar with the "song." It is a well-known children's rhyme which goes:

Twinkle, twinkle, little star.
How I wonder what you are,
Up above the world so high,
Like a diamond in the sky.

There may be music, but usually it is just spoken.

In the film, just after Little Ann jumps out of her brother's truck, the two men drive off and leave her in the middle of the "killing fields," just as it begins to get dark. Ann starts repeating the poem over and over to herself, to give herself courage. She doesn't actually sing.

If you want to hear her sing, watch The Poker House, at the end of which Chloe and her two sisters, played by Jennifer Lawrence and Sophia Bairley, sing along to a recording of Marvin Gaye. That's a very fine film, BTW, though parts of it are hard to watch, Chloe's scenes, including the last one, have been compiled in a couple of Youtube videos that should be easy to find.

http://redkincaid.com

https://en.wikipedia.org/wiki/Twinkle,_Twinkle,_Little_Star