how did hillbillies and rednecks get their very distinguishable voice?

What hillbillies and rednecks? Oh, you mean people of the Southern US? Is that who you mean? Well every region develops it’s accent over time and many variables are involved in that process…I am Southern and not a hillbilly or a redneck…

Local dialects, or accents, develop within a culture of similar individuals. So though you classify only hillbillies and red necks the same can be said for New Yorkers or those from the Northeast or South. ect.

I have never heard one in person. but I have been to the south, is there a similarity or is this another categorization?

location, surrounding, other people, etc…

If by hillbillies and rednecks you are referring to Southerners,

A lot of the Appalachian region as settled by German, Swedish, Irish and Scottish immigrants. Since Appalachia has a terrain that is similiar to those countries, it makes sense that immigrants would favor the area.
Over the years, the languages melded together into the dialect that has become recogniazable. The term “hillbilly” by the way is somewhat derogatory, referring to the popularity of the name William as a christian name at one time. ( i.e. William Joseph Conrad = Billy Joe Conrad, ) They prefer to be called “Mountain Folk”. Much of the modern Appalachian dialect has words that are not far removed from their origins. For example,
Youngin ( a child ) is a phonetic spelling of the German “Jungen” (a boy).
Kith and Kin (friends and family) seems to be derived from Scottish and German words of similar meaning.

In the early days of radio, Applachian folk ballads were very popular. The lyrics of these songs told of the hardships of life, and the joys of love in the region, all in the local dialect. The Nashville clear channel readio station, WSM, capitalized on this popularity by starting a weekly radio variety program, “The Grand Ole Opry”.

The term “Redneck” originally referred to low-wage farm laborers. Since bending over to harvest crops in the sun would result in a sunburned neck, these people were called rednecks.
During the great depression, the Grand Ole Opry provided weekly entertainment to these prople and the popularity of the Appalachian music resulted in some mixing of the mountain dielect into the common southern dialect.