The Line of Sleep

Sue works at a missing children's center. One day, she starts seeing a missing child. With that, Sue starts sleepwalking again. She hears a woman singing in her dream every day. Without knowing who the woman is or what song it is, she wakes up from the dream to find herself in a strange place, and then the song stops. And the missing child is in front of her. Sue chases the missing child and gradually gets closer to the song. Who is singing that song?
(Source: JIMFF)