When you listen to recordings of the "old time" style of Appalachian music that preceded bluegrass, one thing you notice is that the vocals have a droning, nasal sound that's very seldom heard these days. Here's a good example.* At first, I took this to be an artifact of the recording technology - similar to the distortion we see in old movies, where everyone walks in a jerky-jerky way due to skewed frame rates.
Today, though, I was listening to a fairly modern recording of an old time musician, a tune she sung at an advanced age with modern recording equipment, and realized this was a distinct vocal style. My guess is that it was born out of an effort to project the voice in public performances before the advent of amplification.
Amplification has actually affected the way people sing, and what we believe a "normal" signing voice should sound like. Searching for examples, I found this article, which compares videos of heartthrob crooner Michael Buble, singing a tune with the usual application vs projecting it to reach a crowd without the benefit of a microphone. Pretty neat.
* This site, honkingduck.com is amazing if you have any interest in old time musical forms. It has hundreds of mp3s of tunes digitized from old 78 rpm records, and you can spend a ridiculous amount of time just browsing the archives.