Midwestern here, of the rather generic TV American accent variety.
It was explained to me in Bethel that the WT recordings and most "worldly" broadcasting is done using the midwestern accent and a lot of native Canadians, because their pronunciation is so readily understandable to all other English speakers. Most discernible is pronunciation of the Rs, th (both of them), d, ch, sh, wh, but also the vowel sounds. As BOC mentioned, we don't pronounce caught and cot the same. That's something more Southern, I think.
Just my opinion, but I think the accent evolved in my area because more than a century ago it was settled by Americans moving in from the East and the South as well as new immigrants from Germany (lots of them), Netherlands, Scandanavia, and France to that rural area, and Irish, Italians, etc. in the nearby urban areas. The local accent evolved in such a way that these people could understand one another and absorb some of their previous speech patterns.
And another opinion, I think religion has an impact as well. For example, when Baptists and Pentecostals get in religion mode, they seem to ratchet up the Southern sound. Others take on various self-righteous tones. I think the sound of Rutherford, Franz, and Knorr preaching was an effort to sound more like Russell than of anything "normal". I think I heard lots of brothers and newer GB that seemed to fake an accent to sound more like these dudes:
Chuck Russell:
https://www.youtube.com/watch?v=B5tgGpynaac
Joe Boozerford:
https://www.youtube.com/watch?v=MHkZ0RfvHQ0
Freddy "The Oracle of Brooklyn" Franz:
https://www.youtube.com/watch?v=yBwKX-5qXT0
Nathan Snorr:
https://www.youtube.com/watch?v=LjlCRwJ1IVw