He played that sad raggy tune like a musical fool (13)
"Raggy" is a combination between "ragtime" and "raggedy" and implies poverty. Way back in the day, the Fool (think court jester) was this guy who would sing songs and tell jokes for money. He was the Duff Man (from The Simpsons) of the Renaissance; like Duff Man, he would go through bouts of melancholy.
"Ain't got nobody in all this world" (19)
Loneliness is a pretty universal feeling. The solution is to reach out to those lonely people right next to you and share a moment. The singer is sharing his loneliness with his audience, and everyone feels better as a result. As the saying goes: "Misery loves company."
"And I can't be satisfied" (26)
The inability to be satisfied is a guaranteed way to be miserable.