yes. this is a metaphor. you should, however, notice sthing. Metaphor is not just a matter of words. A metaphor can go beyond the linguistic level(a simile usally doesn't).
in the present case the metaphor is built by blending the concept of music with the concept of innocent voices of kids. this will 'surprise' the reader with a new aspect of music where art is about innocence and no longer well elaborated notes.