The title of this poem is pretty straightforward. It's all about a dude who hears the voices of Americans singing as they work (Whistle While You Work, anyone?) , and it's called "I Hear America Singing"—nothing too tricky about that.
But before we move on, let's just note how cool it is that all these different people, from carpenters to seamstresses to ploughboys, are singing while they work. Music is something that unites us all, whether we're working on a dock, plowing a field, or taking care of the kids. In Whitman's America, music is universal.