DVD sales may be slowing, but at least one under-the-radar video niche is actually taking off: the unauthorized DVD music biography. A home-video version of Behind the Music-style documentaries, unauthorized DVD bios are made quickly and cheaply to piggyback on more famous Hollywood releases. Prime example: On Nov. 15, Highland Entertainment released The Unauthorized Biography of Johnny Cash, well-timed to cash in on the release of Walk the Line three days later. Granted, the ''unauthorized'' tag lends these titles a somewhat shady aura, which means mainstream distributors pretty much steer clear. But that hasn't deterred buyers: 2004's Eminem: AKA and 2002's Tupac Shakur: Thug Angelhave each sold more than 100,000 copies and this despite a lack of hit songs or extensive artist footage (there is naturally zero cooperation from the musicians or their labels). To wit: The recently released DVD 50 Cent: Refuse 2 Die doesn't feature, say, ''In Da Club'' or interviews with Fitty, although his grandfather and tangential figures appear as talking heads. Still, there's a plus side to working without the artist's approval, says the disc's director, Mike Corbera: ''I'm giving fans the straight dope.''