Performing on the Late Show with David Letterman in 2008, Pete Seeger did the unthinkable: he waved his hand at the other performers and halted his band mid-song. For a moment, his backing musicians fumbled awkwardly at their instruments, watching the singer for cues.

"You know what, you can sing this chorus with us," Seeger told the studio audience. He began teaching the lyrics to his new tune, "Take It From Dr. King," an upbeat, syncopated number that memorializes the Civil Rights Movement and attests to the power of music to aid social change. After an impromptu run-through, the band struck up again, and the audience pitched in on the chorus: "Take it from Dr. King / You too can learn to sing / So drop the gun."

Seeger, who's 91 years old, has been enlisting audiences in musical and social endeavors since FDR was in office. He first gained national attention as a member of The Almanac Singers, a largely overlooked collective he co-founded with Woody Guthrie and Lee Hays in 1940. The band played at pro-labor events and union rallies throughout the country, leading songs like "Union Maid" and "Which Side Are You On?," as well as Communist-friendly ballads from The Little Red Songbook. Many of his Almanac-era recordings are noteworthy for having true choruses: on the refrains, Seeger's tenor is effaced by the uplifted voices of other singers. It's as though the recordings themselves are meant to inspire participation, urging the at-home listener to join in.

In 2010 the singer is still working to preserve singing's centrality to active American citizenship. "I still lead songs all the time," Seeger said during a series of telephone interviews this month. "I give words to the audience." On some of his more famous tunes, like "Turn, Turn, Turn," and "If I Had a Hammer," he said, "I barely sing the words at all. I just shout out the words before a verse, and the audience does the rest." Seeger sees musical participation as a key part of social participation, and worries about how our culture has outsourced the task of musicianship to a small handful of expert entertainers. "In 1910," he said, "John Phillip Sousa wrote, 'What will happen to the American voice, now that the phonograph has been invented?' And it's true—parents don't sing lullabies to their children anymore, they'll put them in front of the TV to fall asleep. Men used to sing together in bars all around the country—now there's a TV or loud music there instead."

After spending an hour conversing with Seeger, a 160-GB iPod begins to look less impressive: the man is a walking repository of decades worth of commercial and folk music from around the world. He sprinkles his language with snippets of songs in different languages, moving easily between singing and speech while he talks. Considering many Americans struggle to remember the important phone numbers they've digitally stored, Seeger, who doesn't use a computer, is a Luddite monument to the human capacity for mental recall. And most of the songs he knows have been transmitted to him person-to-person, without electronic technology: "I have never in my life liked to listen to records," he said. "I'll listen if I'm curious about what a song is, but I don't like to have background music in the house. I can't do anything else while I'm listening to music. I'll have to try and play along with it." Even alone in his own home, Seeger makes music participatory.