I would like to start off this post by posting the official definition of literature:

noun

1.

writings in which expression and form, in connection with ideas of permanent and universal interest, are characteristic or essential features, as poetry, novels, history, biography, and essays.

2.

the entire body of writings of a specific language, period, people, etc.:

the literature of England.

3.

the writings dealing with a particular subject:

the literature of ornithology.

4.

the profession of a writer or author.

5.

literary work or production.

6.

any kind of printed material, as circulars, leaflets, or handbills:

literature describing company products.

7.

Archaic. polite learning; literary culture; appreciation of letters and books.

And now a few quotes that I think relate to this topic:

     "Where words fail, music speaks."

                              -Hans Christian Anderson

     "Music expresses that which cannot be put into words and that which cannon remain silent."

                                                                                       -Victor Hugo

     Music is literature because it tells a story just like any writings. A book tells a story in words whilst music tells a story in melody. Music can be slow which puts off a sad tone, or fast which puts off a happy or mad tone. A novel, a rap, a song, and a symphony are all related because of this concept. Like literature, music can use words to tell a story. It's crazy how much singers/songwriters and authors have in common.