Just reading the title I expert the poem to be set in the country, because a reaper generally works in a field.
One interesting element that may attract the attention of the reader is the use of the adjective "solitary": The choice may anticipate a melancholic atmosphere or a sad one.

The speaking voice invites the passer by to look at a girl who is working alone in the field in the Highlands in Scotland; while she is reaping she's also singing, and the speaking voice seems to invite the passer by not to disturb her.
The sang is melancholy and seems to fill all the valley, also creating an echo.

The poem is organized into four octaves: in the first one the poet introduce the scene, adding descriptions of the beautiful landscape around the lonely reaper, and he starts talking about the sad strain the girl is singing. In the second stanza he compares her to the two birds that usually are supposed to have a good voice, the Nightingale and Cuckoo-bird, judging her a better singer.
In the third octave there are some hypothesis about the theme of the reaper's song: in the poet's opinion it may be a song about some old battles, or about familiar matter. However, the strain contains an undeniable sadness.
The last stanza reports the emotions the poem feels looking the singing reaper and walking away from her, up on the mount.

The description of the Scottish Highlands is well underlined by the use of open vowels for the words, that bring the sensation of vastness to the imagine of the huge valley.
There are two types of rhyme scheme in each stanza: the first four lines are arranged in an alternate rhyme (ABAB), while the last four have a kissed rhyme (AABB): the rhythm created reminds in the reader a musical scheme, a song, like the one the reaper is singing.
A different sensation is given by the sound of line 29, in the last octave: the repetition of the sounds "l" and "s" suggests the situation of silence and motionless the poet is into when he listens to the girl's song.
There are a lot of exclamation marks in the first part of the text: the reader has to stop upon them, at the end of some strong lines. The pauses anticipate the next lines, usually

There are two main semantic field in the poem. The topic of the first is the country: the use of words like "field", "reaping", "grain", "sickle", gives a first materialistic view of the poet. But in the other octaves there are a lot of words linked to the semantic field of sad emotions: "old", "unhappy", "far-off", "humble", "sorrow", "loss", "pain". This word choice brings to the first imagine of country and calm habits a deeper impression, an hidden sadness.

In conclusion, maybe the poet wants the reader to think how normal and predictable things like country work hide a universe of secret feelings that are not immediately perceivable and that can be noticed only if they are heard alone in the silence.