Music and literature were born together.
The ancient greek poems (lyrics) were written to be sung, not read, and that's literature (ex: hymn, ode, ecloga...)
In Middle Age, the rondo, the song, the cantiga, the madrigal, the ballad were written to be sung, not read, and that is literature.
The minstrel and trobadours in Middle Age were singers, and that is literature too.
So, what about the opera? It was based uppon literature, but it's sung.
Could we define opera as a kind of literature?