Over the course of the film (set in 1937), several characters comment on the public's increasing desire for "old-timey" music. Did such a shift occur in the late thirties? What was the non "old-timey" music the public was rejecting? Was this a national phenomenon, or was this only in the South (where the film is set)?
The real peak of old-time music was probably in the mid 1920's, when Vernon Dalhart's recording of The Wreck of the Old 97 became one of the first records to sell a million or so copies (maybe a little ahead of Gene Austin's My Blue Heaven). That was also the time when the Grand Ole Opry (Nashville) and the National Barn Dance (Chicago) became very popular radio programs, etc.
By the late 1930's, there were many elements of modernization or change going through country music -- Western Swing, electric guitars, modern themes in the lyrics, etc. W. Lee O'Daniel whose name was used for the governor's in the O Brother movie, was a radio announcer and Western Swing band leader who was elected governor of Texas in 1938 on a platform of "Pass the biscuits, Pappy." Albert Gore, Sr, a fiddler from Tennessee, was elected to the Congress the same year. Also in 1938-39, the Carter Family, who had been very popular old-time recording artists for more than a decade, were regularly on the air on a very powerful Mexican radio station near Del Rio, Texas, that claimed a signal of 1 megawatt and could be heard just about coast-to-coast in the US.