This is from Riley Baugus, who built the banjo seen in the film and was a dialect coach, and is a superb Round Peak banjo player who learned from Tommy Jarrel. I got it from the alt.music.country.old-time newsgroup.

I did build the only banjo that you see on the screen and I am the
singing voice of the character Pangle, played by a wonderful actor
named Ethan Suplee. The other musicians involved are folks like
Allison Krauss, Dirk Powell, Tim O'Brien, Tim Eriksen, Norman and
Nancy Blake, Stuart Duncan, Mike Compton, and Jack White from The
White Stripes. I have the honour of singing a song with Mr. White on the soundtrack CD and in the film. Sting, Elvis Costello, and TBone Burnett wrote songs that are in the film and I think they fit perfectly. >
Cheers
Ray