The speaker is telling his audience, that he heard the musician play. Don't people usually say that they saw a band? The speaker is engaging with the musician through sound, and the reader has to engage both through sight (reading).
He made that poor piano moan with melody (10)
A great musician can carry on a whole conversation with his instrument. Jazz musicians do this all the time. This musician is a virtuoso, meaning that he has total mastery of his instrument.
"I's gwine to quite ma frownin'" (21)
By writing in dialect, Hughes is communicating on an unusual frequency. This might put off some readers. If you struggle with it, just read it out loud, because it is spelled phonetically (according to sounds). The thing to take away here is that language provides a lot of room for improvisation, and breaking the rules of Standard English can show you the way towards new forms of self-expression.
He played a few chords then he sang some more– (24)
African and African American music often have a reoccurring pattern called call-and-response. Here is an example of how a singer and an instrument act like two separate voices. The "voice" of the piano calls out a few notes or chords, and the singer responds by singing back to the piano in a similar melody. The two "voices" communicate back in forth in a melodic language.
"I ain't happy no mo' And I wish that I had died" (30)
Did you ever need to get something off your chest and it came out exaggerated? The singer is being dramatic here. He's hit rock bottom. By communicating this to his audience, or just saying this out loud, he takes a load off his shoulders and feels less weary than before.