If he did a vanity film of "Guys and Dolls" playing several of the roles, I could live with that. Although now in my head, I'm just hearing Peter Griffin sing "Sit Down, You're Rockin' The Boat," and Brian (his natural voice) and Stewie duetting on "Guys and Dolls."
I believe he actually has perfect pitch. I would love for Family Guy to do a send up of various musicals (a la what they did with the Star Wars franchise).
I remember hearing somewhere that he did want to do that with The Sound of Music but hasn't been able to work out a deal with the Rodgers and Hammerstein Organization.