"When Lilacs Last in the Dooryard Bloom'd" isn't your typical nineteenth-century elegy. By virtue of free verse alone, Whitman's poem sounds fluid and open as one line often spills into the next (and some lines even run past the printed margin). That's not to mention the lines themselves, which often have one subordinate clause after another, making the final image pile up with details, meaning, and feeling, like this one in Section 16:
As low and wailing, yet clear the notes, rising and falling, flooding the night,
Sadly sinking and fainting, as warning and warning, and yet again bursting with joy. (189-190)
Phew. Try saying that in one breath. We dare you.
We're often flooded with these sorts of extended metaphors, jam-packed with extensive description. The effect on the poem's sound here is one of a frantic energy, with details piling up and the line length dragging on as the speaker struggles to make sense of death. The amount of physical force (breath and voice) needed to sound these lines out is far more than your typical poem. In putting these lines together in such a fashion Whitman is impressing on his reader the urgency, and energy, of his poetic attention.
More specifically, all of the free verse in the speaker's "ever-altering song" sounds true to life in the landscape of grieving and death. We get to hear frequently changing landscapes, which also add to the broad scope of the poem's sound, as things sound pretty, airy, and hopeful one minute and then mysterious and withdrawn or woeful in the next.
Not surprisingly, these shifts correspond with shifts in the poem's sound. When the speaker is busting out his hopeful song in praise of death, then we get examples of intentional use of sound in the poem. For example, lines 47-48 have some serious alliteration going on:
Blossoms and branches green to coffins all I bring,
For fresh as the morning, thus would I chant a song for you O sane and sacred death.
We get B sounds happening with "blossoms" and "branches" and S words with "song," "sane," and "sacred." Actually, we notice that, whenever the speaker talks about the song for death, we tend to see a bit more attention to the sound in the lines. The hopeful and reverent attitude of the content is reinforced by the singsong qualities of the words in the lines themselves. So we get to bob our heads to the sound play, even as the speaker is singing out his praise of death itself. One way or another, it's a beat we all have to dance to some time.