The characters became singing stars in the film

Grammys host Jon Stewart told everybody to go out and buy it - but many will not need to as it has sold more than four million copies and has been one of the most unexpected hits of recent years.

Variously described as bluegrass, roots, mountain music and old-time country, it contains rustic sounds of harmonies, gospel choirs, mandolins, guitars, violins and banjos.

First popular in the 1920s and 1930s, the styles owe their comeback to the comedy film by Joel and Ethan Coen, which is based on Homer's Odyssey and stars George Clooney.

"The reason for our using so much of the era's music in the movie was simple," Ethan Coen said.

'Involved'

"It is compelling music in its own right, harking back to a time when music was a part of every day life and not something performed by celebrities."

The Coen brothers enlisted veteran musician and producer T-Bone Burnett, who had helped put together the music for their last film, The Big Lebowski.

"Being so heavily involved in roots music, we called him before we'd even finished the script," Ethan Coen said.

Burnett preferred to be known as the "music archivist", but "music supervisor" would have been more accurate as he went about finding the songs for the film with singer-songwriter Gillian Welch.

One of the inspired choices was to revive a tune called I Am A Man Of Constant Sorrow, a folk song from the Appalachian mountains first recorded in 1922.

Anthem

The real Soggy Bottom Boys performed at the Grammys

In the film, it became the anthem that made Clooney's band, The Soggy Bottom Boys, Depression-era stars.

In real life, it was recorded by Dan Tyminski, Harley Allen and Pat Enright and became the anthem that made the soundtrack a hit.

Mr Tyminski provided the singing voice for Clooney on the film, and had to warn his wife, Elise, the first time they watched it that when Clooney would sing, she would really hear her husband's voice.

"Your voice coming out of George Clooney's body? Dan, that's my fantasy!" she replied.

Other musicians to feature on the CD included Emmylou Harris, Alison Krauss, the Whites, the Cox Family, John Hartford, and 75-year-old Ralph Stanley.

Spin-offs

While the film was an arthouse hit, nobody expected its music to go on to have so much success that it has taken on a life of its own.

First there was an O Brother concert in Nashville, then a documentary that had its own soundtrack, a United States tour and an O Sister album.

Mr Burnett also won a Grammy for a follow-up CD, Down from the Mountain.

And it has all happened with little radio airplay.

It may not be chart material - but that may be why so many people like it.