What whitman has in mind here are not the actual work songs associated with various trades and kinds of physical labor but something more subtle. what would you say is the real theme of this poem--what is the speaker saying about the american people?

The theme of the poem is the diversity of the American people should be celebrated, and he says that each person should celebrate self-expression (the worker are the backbone of the country) .

Explanation:  

Walter Whitman was born in New York, worked as a volunteer nurse, was also a great poet, Whitman apart from being known for his interest in helping people was recognized as a prestigious writer. He was a controversial poet who liked to publicize his political opinions and for various topics in his writings.

Whitman spoke in his poems of many political situations; for example, among one of his writings is the "Song of America," in which he refers to the importance of each of the people who form the United States and that each individual has the right to express their opinions freely.

I hope this information can help you.