We think this poem has an eerie, echoey sound to it. Part of this has to do with the rhyme scheme: each stanza has two end rhyme sounds that repeat. In the first stanza, the rhymes are what we like to call "ober" and "ere" sounds. Here are the "ober" sounds: sober, October, Auber. And the "ere" sounds: sere, year, Weir.
The echoing sound also comes from lines that are almost-but-not-quite repeats of other lines, like a distorted echo. Check out this example from lines 2 and 3:
The leaves they were crispèd and sere—
The leaves they were withering and sere;
The past sounds are repeating, but with slight differences.
That fits the action of poem really well. Just think about it. The poem is about a guy who's sort of repeating his past actions. Though the speaker doesn't quite remember it until the end, he's been "by the dim lake of Auber/ […] In the goul-haunted woodland of Weir" before. Exactly a year ago, in fact. Both times, he ended up at Ulalume's tomb – the first time to bury her. So just like the sounds are repeating, the speaker's actions are repeating too.
The sound is especially cool at the very end of the poem. At line 91, the speaker's memory is rushing back. At this point we get an echo, or repetition, of the description of the lake that we first experienced at the beginning of the poem. In a way, when we read this echo of a previous line, it's as if our own memories were coming back too. Check it out:
It was hard by the dim lake of Auber,
In the misty mid region of Weir—
It was down by the dank tarn of Auber,
In the ghoul-haunted woodland of Weir. (lines 6-9)
Well I know, now, this dim lake of Auber—
This misty mid region of Weir—
Well I know, now, this dank tarn of Auber—
In the ghoul-haunted woodland of Weir." (lines 91-94)
Pretty cool, huh?