The sound of her voice is plain; it doesn’t have any special qualities. He likes to hear her speak because he appreciates her conversation.
Nevertheless, this marks a change in the meaning of the poem. It follows the traditional form, with the first eight lines establishing a premise, and then the next six lines turning that premise around. In this poem particularly, the previous lines talk about how Shakespeare’s companion is nothing special, while the lines following tell us how Shakespeare still loves her with all of his heart.