I think that the sense of wondering about the song that the reaper sings helps to create an aura of mystery in the poem. It is from this that the speaker, presumably Wordsworth, is able to construct the sense or aura of mystery that surrounds the woman. Of what does she sing? What is the meaning? How is it reflective of her own condition? At the same time, the song enables a sense of wonderment to transpire about her own state of being? From where does she hail? Is she happy? Even, if she is single are all questions that Wordsworth, through the speaker's voice. are able to construct and generate. This helps to establish a sense of aura of mystery surrounding the poem. It is one in which there is uncertainty about the woman, about the speaker, and about the conditions enveloping each. The only certainty is the experience, itself, and its transformative impact on the speaker, and thus, on us. The experience is the only certainty and all else is subject to myster, wonderment, and a sense of amazement of being. This is a very Romantic idea being put forth.