Question: I download lots of lectures from iTunes U. The problem is, they usually come with video and there is no audio-only version for many.

How can I, using any software available for Snow Leopard, free or paid, extract the audio from a series of video files and end up with a series of audio files?

Any ideas?

Answer: 
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.
I use AudioHijack Pro it costs money though. I can rip audio from DVDs and Internet streams.