Now I can't not imagine that he sounds like Matthew McConaughey.
TG: your gross man-bro crush on matt macconahay is an unsavory thing to behold
Also John's voice is and always will be Michael Guy Bowman. You can't prove me wrong.
Headcannon time:
John is Octopimp/Micheal guy Bowman, whichever one is nerdier. In my experience (I think John lives in Washington) Those northwest don't have that significant of an accent, though I could just be talking out of my ass.
Rose is the "mature" (and later legit mature) voice that tries to stay (and later does so effortlessly) in the "silky shit and such" type of voice. I'm not sure what the accent from that area would be, but I'd like to think Rose is so controlled that she'd have trained an accent out of herself.
Dave is the smooth coolguy, and it can go one of two ways. One is Matthew McConaughey super obnoxious accent, the other being what most people in the urban south experience (the accent is mostly gone, but still there, and especially stands out when you're stressed). Also, Dave is an ironic fan of the Aggies because fuck not loving football.
Jade is the one who would be the most fun to figure out. I always figured that, since Grandpa Harley is hella dead, she would pick up her accent from whichever traders happen to be in the area (ie people who deliver the mail. etc.). So, Jade either gets the slightly southeast asian/australian accent, or the thick argentine accent (the more I think about it, the more I adore Dave trying to decipher why all her y's are j's, stuff like that)
Oh man I need to write something for this
FAKEEDIT: Also I just got this hilarious image of Dave trying to hold an actual conversation with an A&M fan and just failing like
"Nah man I think Kenny Hill's got a great season ahead of him"
"yeah man he passes the ball and shit its fuckin crazy"