This is one of Wordsworth's famous poems that describes the sight of a female reaper reaping in fields. Wordsworth is so entranced by the sight that he stops and stares and reflects on the sight. The words that your question refer to come in the first stanza, and presumably are mentioned to any who are with him or who may be passing on the same path. Note their meaning: Wordsworth is so wrapped up in the vision of the reaper and her singing that he doesn't want any interruption to alert her to his presence and to break the spell that she casts:
O listen! for the vale profound
Is overflowing with the sound.
Thus, although it is not specified who the words are addressed to, we can infer that they are spoken to either companions who are with Wordsworth or any other rambler who happened to be walking in the same spot.