If you stretch the criteria to songs that have references to boxing there are a ton in hip hop. The Essence by AZ ft Nas references Hagler Hearns and maybe some more, Wu Tang Clan- The MGM, Ghostface Killah- The Champ. I know there are a ton more hip hop songs with at least a few lines about boxing but my mind is blanking, Vinny Paz of Jedi Mind Tricks took the name from the boxer but I don't care for the groups lyrics. On a more mellow non hip hop side Bill Callahan of Smog has a song Sycamore that has some boxing references and the Smog song Feather by Feather has the lyrics "Its Ali vs. Clay/Both pummeling away/A champ always fights themself."

More than a testament to how many musicians are boxing fans I think they see the inherent metaphors, in such short and profound description, that the sport provides. Also boxing is so ingrained into American history and culture that the sport provides a wealth of material for artists, especially those looking back into history for subject matter.