For decades jazz cognoscenti have talked reverently of “the Savory Collection.” Recorded from radio broadcasts in the late 1930s by an audio engineer named William Savory... only a handful of people had ever heard even the smallest fraction of that music, adding to its mystique. After 70 years that wait has now ended. This year the National Jazz Museum in Harlem acquired the entire set of nearly 1,000 discs, made at the height of the swing era, and has begun digitizing recordings of inspired performances by Louis Armstrong, Benny Goodman, Billie Holiday, Count Basie, Coleman Hawkins, Lester Young, Bunny Berigan,...