When we read this poem out loud, we kind of want to sing it. That's not a surprise, of course, given that it was, you know, written to be sung. The poem has such strong rhythms that it pulls us right into sing-song land. But don't take our word for it. Let's check out the first few lines:
Lift every voice and sing
Till earth and heaven ring
Ring with the harmonies of Liberty
Let our rejoicing rise
High as the list'ning skies,
Let it resound loud as the rolling sea. (1-6)
How can we not sing those lines? For one thing, all of the rhyming gives a strong sense of melodic sound to the poem (check out "Form and Meter" for more on that). There's also a pattern of iambic meter (again, "Form and Meter" has you covered there).
Sound-wise, however, there's a ton of consonance going on here. If you read those lines out loud (go ahead, we'll wait here while you do), you should hear a lot of S sounds flying around. In fact, S sounds pop up all through the poem. This gives the poem a sense of sonic smoothness and creates a pleasant sound echo.
In addition to those S's, we get some cool alliteration in a line like, "We have come over a way that with tears has been watered" (17). Dig those W's? This is just another example of pleasing sounds strung together in this poem.
So, what gives with these techniques? Well, this poem is a kind of celebration. It acknowledges racial progress in the face of adversity. So, these pleasant sounds act kind of like our ear's reward, subtly punctuating the lines with triumphant, sonic flourishes. Ultimately, all those good sounds add to the song-y-ness of the poem. Heck, even without music, we'll find ourselves singing along.