He played that sad raggy tune like a musical fool.
Ragtime had been around for about twenty years when Hughes wrote this poem, and it was pretty popular across cultures. Scott Joplin, known for originating ragtime, was a classically trained musician, but ragtime was popular with many people. By making the music raggy, the speaker is connecting the music to the common people that Hughes was so fond of.
"I's gwine to quit my frownin'" (21)
In this line, the grammar isn't standard and the spelling mimics a very strong accent. This puts some cultural distance between the singer and the speaker, who uses Standard English. However, the dialect lyrics could be for show, because the musician thinks that they make the music sound authentic. He is giving a performance. You don't talk to your grandma the same way you talk to your friends, do you?
While the Weary Blues echoed through his head. (34)
The musician has the song stuck in his head. It was such a catchy melody that he can't get it out.