"The Solitary Reaper" is a poem all about sound. More specifically, it's a poem about how sound can be interpreted even when there are no intelligible words. The reaper herself, for example, sings a song in a language the speaker can't understand, but he can definitely tell that it's a sad song, and he's incredibly moved nonetheless. The woman's song, in fact, is a lot like opera in this regard (we cover this idea further in our "Why Should I Care?" section).
Speaking of what things sound like, the speaker himself sure sounds a lot like a tour guide, somebody who would accompany you on your journey through the rural environs of Scotland and point out all the local sites of interest. Just look at all those commands in the first stanza of the poem. They are the hallmark sounds of a tour guide: "Behold her" (1), "Stop her, or gently pass!" (4), "O listen" (7). We kind of imagine him talking just a little bit like this guy.
Not only does this poem sound like something you'd hear from a tour guide, it also sounds like something you'd hear from a tour guide who is really into what he's talking about. In the middle of the poem, for example, the speaker raves and raves about the woman's song, making long, elaborate comparisons about how thrilling the woman's song is: "A voice so thrilling ne'er was heard / In spring-time from the Cuckoo-bird, / Breaking the silence of the seas / Among the farthest Hebrides" (13-16).
Part of that enthusiasm is reflected in the sound of this line, too. Hear those repeated S sounds ("so," "spring," "silence," "seas")? That's some alliteration for you. S alliteration, in fact, is elsewhere in this poem, too, like in stanza 1("singing," "solitary," "single," "Stop," etc.). The result is a smooth, pleasing sound—much like the speaker would have been experiencing when he listened to the reaper's song.
That song has a powerful impact on our speaker. Even though our tour guide really wants to move along, he can't, or he doesn't want to. This is why he tosses off all those lengthy comparisons in the second stanza, and wonders aloud at length in the third stanza about what the woman's song is about. This also explains why there are so many long vowels in the poem (that's called assonance in the poetry biz). The long A, for example, is everywhere—"strain" (6), "Vale" (7), "Nightingale" (9), "Arabian" (12), "lay" (21), "day" (22) "pain" (23), "again" (24).
So why so many? Long vowels are, well… longer. The speaker uses words that have them because, in a subtle way, they take longer to pronounce. If you use a bunch of words with lots of long vowels, then that means your mouth will linger on them for a little while before moving on. Those vowels, you could say, are the sounds of a tour guide who wants to hang out for a bit and drag the whole, groovy experience out.