If I bookmark a video, it may be months before I watch it, if ever, but audio I get around to more quickly. I have more time for listening to audio than watching video, so I’ve started going through my video backlog by downloading the files and extracting the audio to put on my iPod.
I download the video with Video DownloadHelper, a Firefox plug-in, then extract the audio to an MP3 file using Sound Converter. (I assume DownloadHelper works cross-platform, but Sound Converter only runs on Linux.)
DownloadHelper has an option to download and convert in one step, but I haven’t gotten that to work. Also, you can use
avconv to convert video to audio, but they have failed more often than not in my experience. So far Sound Converter has always worked for me.
Update: TubGet will let you enter a video URL and download an MP3 file. I seems to work OK, and is faster than downloading video. There’s also ListenToYouTube but apparently it only works with YouTube while TubGet also works with Vimeo and others.