The sound of this poem is long and lingering—almost lazy, at times. As discussed in "Form and Meter," Keats specifically chose a slow, time-consuming meter, and the way he writes that meter slows it down even more. His sentences are super-long and frequently have a bunch of subordinate clauses with lots of semicolons and dashes, just to draw things out even more.
For instance, check out stanza 26, where Madeline's getting ready for bed:
Anon his heart revives: her vespers done,
Of all its wreathed pearls her hair she frees;
Unclasps her warmed jewels one by one;
Loosens her fragrant bodice; by degrees
Her rich attire creeps rustling to her knees:
Half-hidden, like a mermaid in sea-weed,
Pensive awhile she dreams awake, and sees,
In fancy, fair St. Agnes in her bed,
But dares not look behind, or all the charm is fled. (226-234)
The first thing that you notice is that this entire stanza is basically one sentence, when it could have easily been made into six or seven. With a whole lot of semicolons and some deep breaths on the part of sucker who's reading the poem aloud, though all of these little movements (Porphyro's revival, Madeline letting down her hair, taking off her dress, etc.) become one long, unbroken, and achingly slow sequence leading up to this ending image of Madeline, naked and envisioning St. Agnes in her bed.
So why this slow-sounding approach? "The Eve of St. Agnes" is frequently described as a celebration of the physical senses. In addition to describing lots of sensory things—sounds, colors, smells—Keats achieves this Hooray Sensory Experience thing by taking his sweet time. The poem doesn't rely much on devices like alliteration—in fact, it's almost anti-alliterative at times. All of the words are made up from such different sounds that it slows you down even more, and you linger over each word.
As though a tongueless nightingale should swell
Her throat in vain, and die, heart stifled, in her dell. (206-207)
Can you imagine saying that fast? Probs not.