I think that there are a couple of levels of significance to the speaker's question. On one hand, it becomes evident that the speaker, presumably Wordsworth, travels alone. While the reaper in the field is "solitary," the speaker in the poem is also solitary in that there is no one else with him and no one else is able to provide clarification for him. His questions and his journey are individualistic and he is to travel on his own. In asking the question, this condition is highlighted. Another layer of significance here is that the speaker lacks the comprehension of the song, and thus is able to construct multiple meanings as to the meaning of the song in his own mind. This helps to bring out a type of "negative capability" element to the poem, a heavily Romantic idea. The speaker recognizes that he does not understand the song being sung and "no one will tell what she sings." In this, the speaker is able to construct his own meaning to the song and enable himself to develop his own understanding to it. This enables the moral and ethical imagination of the speaker to expand and embrace that which can be as opposed to what is. There is a certain amount of intellectual freedom and creative energy that is generated by the face that "no one will tell me what she sings." This becomes a source of joy for the speaker, presumably Wordsworth.