The song playing as the Pevensie children leave Narnia, and the movie comes to an end is The Call
The Call is maybe not an obvious fit for this book. (It would definitely be a more obvious choice to use it as a song for something talking about, say, King Arthur's return!) It's the way it starts, though - the description of the slow growth from feeling to battle cry - that makes me think of how Sorcha's determination to save her brothers never falters, no matter how difficult her situation, how that determination even grows stronger as the horrors she goes through pile up around her.
From early on in her task:
"They were dark times, and in the depths of them I would hear an inner voice that said, this task is impossible. Why not give up now? Look, your hands are swollen and ruined, you weep day in and day out, and what have you to show for it? A little spool of ill-spun thread, lumpy and fragile, scarce enough to hem a jacket for a butterfly, let alone a shirt for a man. Surely this task cannot be completed. Besides, how can you be sure the Lady of the Forest did not lie to you? Perhaps this is all some cruel trick, and your labors are for nothing."- to the end, when she faces death rather than break her silence
"One night, I thought, my heart pounding. Only one night, and then my fate would be decided. I had to be strong, I must keep my mind away from fire, and from death. ... I did not sleep at all that night. There must be time to finish. There had to be. Did the Fair Folk set a task, and then make its completion impossible? I could not believe that it would be taken from me, so close to the end. I must finish. I would finish."Sorcha never gives up, clinging to the shirts she has made even as she is tied to the bonfire, searching the skies for her brothers so that she can save them before her work burns with her.
Pick a star on the dark horizon
And follow the light
You'll come back when it's over
No need to say goodbye
You'll come back
When they call you
No need to say goodbye
Books: Daughter of the Forest
The Chronicles of Narnia
Music: The Call