*The* audiobook which changed my life is -- and I know there are diverse opinions on this -- Shelby Foote's The Civil War: A Narrative. Not everyone is a fan of Foote I realize, but his writing style mirrors fiction so closely (after all, he is an author) that it really lends itself very well to audiobooks. His entire 1.2 million word tome is available in audiobook, read by the wonderful Grover Gardner. I have listened to it all the way through once, and am about 1/5 of the way through my second go around right now (I have a morning commute and it makes the time fly by). It keeps the war always on my mind, for better or worse, and I would sincerely recommend it.
If Foote's not your thing, I would second the recommendation for Tony Horwitz's Confederates in the Attic. Also, and I personally own this on audiobook and can speak for its quality, Horwitz's book on John Brown's raid on Harpers Ferry, Midnight Rising: John Brown and the Raid That Sparked the Civil War, is extremely recommendable (if you're OK with something slightly predating the war).
Audio books are wonderful ways to make being in the car enjoyable!