With the word, "Up," and a conductor-like wave of his hand, Bob Boilen draws in the strings, the clarinet or even the accordion for a quick musical interlude on National Public Radio's "All Things Considered." As the program is airing, Boilen, one of the show's two directors, periodically snatches a CD from a stack of about 15 and pops it into the player in front of him. He hands discs over to the engineer, saying things like, "cut four at 1:38."

The result is a fairly satisfied and often intrigued listening audience, some of whom may call and ask, "Just what was that you played between the segment on the WTO talks and the one about mass graves in Mexico?" The answer, for the December 1 edition: a bit of jazz--Tuatara's "Smuggleros Cove"--and a touch of classical piano--John Arpin's rendition of "The Flirt."

Listeners have always taken notice of these musical interludes--"buttons" as they're called--on NPR programs, whether they're during "All Things Considered," "Morning Edition" or "Talk of the Nation." The audience services department alone gets about 30 calls a week inquiring about the music. The tunes are quirky and obscure, sometimes recognizable, often not, all over the lot, and a vital part of the programs. Radio, after all, is sound.

Music is functional, helping to fill time or end a show for local stations, says Boilen, who job-shares his director post with Marika Partridge. But, beyond that, "it's mood, and it's a breath in the show," he says. "It's a place to either...