Since we're reading Rimbaud in translation from the original French, you're going to have to take everything in this section with a grain of salt—er, French salt, that is. Maybe it's sprinkled on some delicious escargot (mmm, snails).
Regardless, to talk about sound in "A Season in Hell" is to dive into a layer cake (sorry, we must have skipped lunch today) of poetic considerations. The original, of course, sounded very...well, French, while the version we have, like any translation, may, or may not, duplicate some of the same effects in English.
As a starting point, though, we can focus on those moments in "A Season in Hell" when the speaker decides to get all formal with us. After all, the majority of this poem is put together with free verse lines (check out "Form and Meter" for more), which try to mimic the flow and sound of conversational speech. All the same, the speaker is not going to let us get away without appreciating his poetic chops. At one point he says, "I invented the colour of vowels! A black, E white, I red, O blue, U green. – I regulated the form and motion of every consonant, and, with instinctive rhythms" (112). With that kind of boasting, you know this guy's got some sound effects he's just dying to drop on us.
Soon enough, he does. He dives into some formal quatrains to render a short pastoral mini-poem, which includes this stanza:
From that young Oise, what could I be drinking,
– Mute elms, flowerless turf, dull sky –
From yellow gourds, far from my dear hut slinking?
A gold liquor that yields sweat by and by. (118-121)
Right away your ears should perk up. In the English translation, we have the consonance of the L in "elms," "flowerless," and "dull" in line 119, followed by the R sounds in the next line: "gourds," "far," "from," and "dear."
But do these sound effects carry over from the original, or are they inventions of the translator? Check it out:
Que pouvais-je boire dans cette jeune Oise,
Ormeaux sans voix, gazon sans fleurs, ciel couvert.
Que tirais-je à la gourde de colocase ?
Quelque liqueur d'or, fade et qui fait suer.
You don't need to be fluent in French (though it helps) to recognize the S sounds in "sans" and "fleurs" (and "ciel" too, by the way) in the second line. And the fourth line features the R sounds of "liqueur","d'or," and "suer." Clearly the consonance is carrying over here to the English version from the French original, proving to readers that Rimbaud knew how to turn a line and craft an echo to catch our ears—both English ones and French ones.