The Solitary Reaper is a poem that acts on the values of Lyrical Ballads by placing praise and beauty in a rustic, natural setting, and by establishing as its source a simple rustic girl. The poem's structure is simple. The first stanza sets the scene; the second offers two comparisons for the music, the third wonders about the context of the songs, and the fourth describes the effects of the songs on the speaker, and it's language is natural and unforced.
The final two lines of the poem, "the music in my heart I bore, long after it was heard no more" present the familiar theme of memory, and the soothing effect of beautiful memories on human thoughts and feelings.
In the review I found on the Internet, the author comments that, "Throughout the course of the poem Wordsworth's voice evolves from being an outsider voice into an insider voice..." I doing so, Wordsworth takes on both a hetero-diegetic and homo-diegetic point of view. In the first two stanzas Wordsworth emerges as an outsider voice observing the "solitary Highland Lass" but continues on to become somewhat of an active character, making the transition from one to the other in the third stanza when he mentions a personal and emotional conflict that he faced, and he questions himself if the battle was humbly fought or not.
It appears to me that Wordsworth is unable to comprehend the girl's song but he is able to appreciate its tone, its expressive beauty, and the mood it creates within him, rather than its explicit content, at which he can only guess.