Does anyone know if there is a web service to extract the audio from
sources such as Google Video, YouTube, etc.?

As anyone who has viewed these can tell you, the video is, most of the
time, optional content. For me, it has been much more productive to
listen to the talks on my mp3 player. Only when I think I _have_ to
see a slide or graphic, do I ever go back to the source video.

For example, Google has a Tech Talk series, Harvard has its
MediaBerkman video series, etc. The latter often has an mp3 audio-only
version that they produce from the video. Google Video and most of the
web video repositories almost never do this. That's why the audio
files from this year's Code4Lib conference, standing along side the
Google Video, was so brilliant.

What I'm hoping to find is a service that, given the url of the video
on the web, returns an mp3 file of just the audio portion. I know
there is desktop software that does this. That's not what I am looking
for. My question is directed toward finding a web service.

An acceptable, even preferrable, alternative to the web service
approach would be, say, an iTunes plugin that, given a video RSS feed,
extracts an audio-only podcast version for the videos in the feed.
This would be ideal, actually.