Keats believes in the power of poetry. In this poem, he isn't praising the content of Homer's work, he's praising the poetic language of the translator George Chapman. A huge part of poetry is its ability to create effects with sound.
The most obvious sound feature in the poem is the rhyme scheme (check out "Form and Meter" for more on that pattern). Petrarchan sonnets have very heavy rhyme patterns; the first eight lines only rhyme with two sounds. In this poem, that's "-old" and "-een." There's a pretty strong contrast between the low, almost shouting sound of "-old" and the high-pitched "-een." The low sounds are repeated consistently and hold the poem's epic tone.
The high sound, in contrast, only technically repeats twice: in the first word "seen" and the last word "serene." The second rhyme "been" is a sight, or eye rhyme —it looks like it rhymes, though it technically does not. "Demesne" can be pronounced "di-MEEN," but is more often pronounced "di-MAIN" (like domain). So those rhymes become less important. This isn't uncommon in Petrarchan sonnets. English isn't a very rhyme-y language compared to those poetic romance languages like French, Spanish, and Italian, so getting that many rhymes is tough and often calls too much attention to the sound.
The final six lines have a simple alternating rhyme scheme with the sounds "-ies" and "-en." Again we have a more high-pitched vowel sound contrasted with a lower one. In this case, the rhyming words tie together nicely. "Skies," "eyes," and "surmise" are all linked to give the reader an impression of wonder and discovery. "Ken," "men," and "Darien" remind us more specifically of the actual explorers. This contrast mirrors Keats's own focus on both the discovery of new worlds and the brave, noble explorers (or poets) who relate their discoveries to the rest of the world.
The poem also reads very smoothly because of all the liquid consonants, L's and R's. Those are the sexy sounds of language, like Barry White... or Yanni. The first line gives us nice, smooth consonance with L sounds: "travell'd," "realms," "gold." Although lines three and four are similar, line two interrupts with harsher T's and K's to emphasize the more structured and formal "states" and "kingdoms."
We also get a lot of S consonance in line 2—and throughout the poem, in fact—which, like Keats' other choice of sounds in this poem—helps to reinforce on a sonic level what he's doing in terms of content. All those smooth S sounds? They create a soft undertone, a pleasing current of sighs that reinforce the awe and wonder that our speaker feels when he encounters Chapman's words.