In some cases, it's not the material in the audiobook which makes it a snoozefest, but the narrator. Several of the liberty-oriented books are read by the same fellow with a monotonous style of speaking.* May not bother some, but his style sends me straight to groggy. After hearing Riggenbach's (spelling?) read of Libertarianism: A Primer, I've avoided other books narrated by him, and will just get the printed copy.
*Actually, I should just say monotonous reading style. His tone in personal conversations might be more lively.
Edit:
Oh! For Rothbard, you should check out the History of Economics series in the Mises library. It's great to hear it straight from Rothbard.
In fact, the lecture series are wonderful to listen to. For the books, I tend to read them, unless it's got a narrator that keeps my attention.
I've listened to a lot of books with my Audible subscription. I'll skip a few of the less lively narrators.
The narrators of Heinlein's books tend to be pretty good:
Friday, The Puppetmasters, Time Enough for Love, Stranger in a Strange Land. The lunar language quirks can be off-putting when listened to and break concentration, so The Moon is a Harsh Mistress is probably not good entertainment while working.
Predictably Irrational, Dan Areily
The Logic of Life, Tim Harford
The Tipping Point and Blink by Malcolm Gladwell
Adventure Capitalist and A Bull in China, Jim Rogers
Meet You in Hell, Les Standiford (About Carnegie and Frick)
Stiff:Curious Lives of Cadavers, Mary Roach (DL'd this after Rillion mentioned it - very glad I did. It was fascinating.)
There are a lot of other books, both fiction and non-fiction, but that doesn't necessarily fit what you're looking for. I also have a lot of books DL'd for listening to on trips with the kids. The His Dark Materials trilogy by Phillip Pullman was beautiful. Full-cast audio production - thoroughly entertaining. Not liberty-themed exactly, but definitely appeals to the more objective way of seeing the world. The Bartameus Trilogy by Jonathan Stroud is good too, and is read by Simon Jones, who is probably my favorite narrator.