The mouth organ and the stringband
The first white settlers of the Appalachians were Irish and Scottish immigrants who moved south along the Blue Ridge from Pennsylvania, bringing the music of the British Isles. Newly freed slaves migrated north from the plantation lands of the Piedmont and Delta regions, bringing with them the polyrythmic music of western Africa and the banjo. It was the medicine shows and traveling minstrels that gave birth to Blues, Bluegrass, and Country music. Black and white musicians playing and swapping techniques and songs. It's not a coincidence that black blues musicians and white hillbilly musicians shared many songs in their repertoire.
One of the instruments that was also a common link was the harmonica. Mass produced, inexpensive, and small enough to transport, the harmonica was a common instrument used by both black and white musicians.
For the most part, the harmonica has fallen from use in country and bluegrass. Charlie McCoy (the modern master of bluegrass harmonica), Jimmie Fadden (Nitty Gritty Dirt Band), Terry McMillan (Nashville's top session player for the past twenty years) are the only names that come to mind nowadays. That wasn't always the case. The harmonica was a common instrument in stringbands of the late 1800s, and early 1900s. Unfortunatley, the harmonica was beginning to lose it's place in the hillbilly band just about the time the record companies started to take an interest in rural music. Hence there isn't alot of harmonica mountain music preserved.
The Woodie Brothers were from Ashe County, North Carolina and didn't venture far from home. During the Great Depression they cut a record or two, but givin' that folks were having trouble putting food on the table, buying records was a luxury. The Woodie Brothers sold only 864 copies of their record. But they knew they would be rewarded "over there", even if they had to hit ol' Satan "on the head with a two by four".