So far as I know there is no good system to do this. (someone correct me if this is wrong). There are lots of service web sites that will do this for you, but they are slow and full of annoying adds and shit like that. The ideal thing would be a Firefox addon that just has a "save audio on this page as mp3".
(ADDENDUM : I guess there is one called "DownloadHelper", but they want money for the MP3 conversion feature. DownloadHelper does seem to be a little better than Unplug in that it has special code for Youtube that identifies the various format streams available so you can download the best one. Amusingly DownloadHelper seems to be very mainly targetted at porn site scraping; it has features like automatically finding new videos from certain sites and putting them in your queue for download. I guess it's the new era version of the automatic NNTP downloader I wrote in my youth. It looks like the FLV's generally have worse audio than the MP4's, so you should prefer them when possible perhaps.).
What I'm doing :
I use "Unplug" to download the .FLV video from Youtube. (* use DownloadHelper to download .MP4 video instead).
I use "MPlayer" to get the WAV audio out of the FLV. The way you do that is :
mplayer.exe -ni -vo null -vc dummy -ao pcm video.flvThen I use the LAME command line build to encode the WAV to mp3. BTW there's a ton of bad old advice out there about LAME command line settings; I use this :
lame -h -V1 audiodump.wav r:\audiodump.mp3but sometimes I wonder if I should use this :
lame -h -V1 --preset extreme audiodump.wav r:\audiodump.mp3which actually makes smaller files usually because it enables some funny filters or something. Dunno.
Anyway, as a compression worker it bother me morally that I am decoding compressed audio from the FLV and recompressing it - I'd really like to just rip the audio stream directly from the FLV and be able to play that, but the fact is that my iPod and my car want MP3's, not AAC or whatever the fuck is in FLV, so it's pretty hopeless.
It's also annoying that I have to then enter the IDV tags by hand. I wish I could scrape the info right off Youtube. In theory you could use a CDDB auto-tagger on the MP3 to do it, but here are two problems with that : 1. I can't find a command line CDDB auto-tagger out there, there are several of them but they are all GUIs, and 2. even if you fire up the GUIs, CDDB seems to just spew fail all over itself.
ADDENDUM : if you don't need MP3's it is actually very easy to get the audio track directly to an M4A which can be played by iPods & most media players these days. Use mp4box ; I suspect there's a more direct command line that will work but this is the way that I've succeeded :
mp4box -new out.m4a -add gremlins.mp4#audio -ipod