"I Hear America Singing" is basically an 11 line list (for more on this form, see the "Form and Meter" section). When you read it out loud—and we recommend that you do—its listiness really comes to life. As the lines get longer, and our pile of laboring Americans accrue, the poem keep upping its excitement levels.
It says: We have carpenters! And hatters! And ploughboys! And mothers! More, more, more! The poem keeps building in excitement (and, let's be honest, in volume) until its final lines, which we read out loud as a powerfully strong statement: "Singing with open mouths their strong melodious songs." Did you hear all that assonance and consonance in that final line with the chiming of "strong" and "songs"? Did you catch the alliteration of all those repeated beginning S sounds ("Singing," "strong," and "songs")? The whole poem builds to this moment of powerful melodiousness, which is underscored by the many chiming sounds in this line. Nice work, W.W.