What do the aural properties of the poem "The Idea of Order at Key West" contribute to its meaning?
The aural properties of (particularly modern) poetry can be hugely important in creating the effect of the poem as a whole, sometimes working unconsciously rather than literally to create meaning.
Stevens' poem is written in iambic pentameter, studded with alliterations and assonances, which recur in various patterns throughout the poem, creating the lilting sense of a tide moving backward and forward: an aural version of one of the poem's key images. I've highlighted some examples of these alliterations and assonances below: try reading the extract out giving each one a little extra stress.
She sang beyond the genius of the sea.
The water never formed to mind or voice,
Like a body wholly body, fluttering
Its empty sleeves; and yet its mimic motion
Made constant cry, caused constantly a cry
The 's' sound which echoes along the first line and in words like "sleeves" also might be read as being closest to the sound of the water lapping against the shore.
Rhyme also gently touches within and at the end of lines, creating the same lilting, rocking, sea-like effect. Have a look at the second stanza, and notice the way one single rhyme sound aurally structures it into a whole. After the internal rhyme (sea/she) in the first line, a treble rhyme ('heard' in line 3, 'word by word' in line 4, 'stirred' in line 5) on the same sound leads to the repetition of 'heard' in the final line.
In short, then, the sound properties of the poem directly reflect its themes.