Rich Larsen of Caponefanclub.com plans to release a recording of Capone's "Madonna Mia" on his Web site Monday, the Chicago Tribune said.
Larsen, of Prospect Heights, Ill., says Capone apparently composed the ballad while serving time for income tax evasion at Alcatraz in the 1930s.
"It's a very touching Italian love ballad," says Larsen. "I think it shows that Capone actually had a softness in his heart and a gentle way about him in certain ways."
"Madonna Mia" is believed to be an ode to Capone's wife, Mae, and is the only song the Mafia chieftain is known to have composed.
An inscribed copy of the sheet music framed alongside a picture of Capone is on sale for $65,000 at the Boston location of historical document dealer Kenneth W. Rendell.