Really, the sound of this poem depends a lot on what language you're reading it in. Remember that this is translated from the original Spanish, so there are things to listen up for with both the original and the translated, English version.
In both cases, reading this poem, with its stanzas, repetitions, and ellipsis-induced pauses, is a necessarily slow process. It isn't difficult, but it is not something you can just cruise through, either.
In English, check out the alliteration, or repeated first sound, in "fiercest face" or those "bloodstained blows are the crackling of bread burning."
In the "slap on the shoulder summons us" the repeated S is all over the place, not just at the beginning of the words, so it's called consonance. In the original Spanish, the line goes "cuando por sobre el hombro nos llama una palmada," which is a great example of assonance, the repetition of a vowel sound; in this case, first we get a lot of O's then a lot of A's. As in: "Oh! Ah! Thanks for breaking this down for me, Shmoop!" You're welcome.
Each repeated sound brings up powerful images to back up the ones the poem mentions directly. For example, the F has you push air out through your teeth like you're working hard or being hit. In the original, too, the F is repeated in that line: "en el rostro más fiero y en el lomo más fuerte."
The B sound is like a blubbering baby—in fact, all that B-ing might make you think of the sound of someone crying. The P sound is the one that's repeated in the original Spanish, and it's very similar to the B, if you think about it. Go ahead, smack those lips and marvel at the similarity of B and P while you read this line in Spanish: "algún pan que en la puerta del horno se nos quema." Quite a lip workout, huh?
The S reminds us of a snake, and after all the religious imagery and sin and suffering, a snake wouldn't be too out of place in this poem. Even though the line about the shoulders doesn't have a repeated S sound in the original Spanish (see above for what it does have), the line about the bloodstained blows does:
Esos golpessangrientosson las crepitaciones.
The poem really manages sounds, and also pauses that it forces on us with punctuation, to create a slow, labored song, like a lament, that really fits with the downtrodden and depressed themes that it's dealing with.