Old question, but one you can do that nobody mentioned is, download a bunch of Video Lectures using Itunes U.
Add the selections to a Playlist in iTunes.
Right click the Playlist and select Burn Playlist to Disc:
There will be Disc Formats : Audio CD. and MP3 Cd.
selecting either of those options will strip just the audio out of the disc. Selecting Mp3 CD will give you the list of Mp3 files you were after.
The only catch is you have to write them to disc first. But it requires no third party tools or downloads and if your car doesn't have an easy audio hookup for the iPhone you can play them right off the CD.