The Philly Sound is going to the movies. The Sound of Philadelphia, a new documentary film, will tell the story of songwriters and producers Kenny Gamble, Leon Huff, and Thom Bell, architects of the Philly soul sound that was a dominant musical and cultural force in the 1970s. The film is now in production with a team that includes Oscar- and Emmy-winning documentarian Alex Gibney. Gamble, Huff, and Bell — collectively known as “The Mighty Three” — have a shared catalog of over 3,500 songs, whose innumerable hits include The O’Jays’ “Back Stabbers,” Billy Paul’s “Me & Mrs. Jones,” The...