There are many hints involving this issue, but having gone through most of them, they all involve purchasing something at some point in the process, which is galling if you only need this facility rarely, so here's a totally free method.
A three step procedure to accomplish this:
- Download the video using your favorite extractor. You cans search for one if you don't already have a preferred tool. If the file doesn't have the extension .flv, then add it.
- Open the video in VLC, then open VLC's Streaming/Exporting Wizard, select 'Transcode/Save to file', then select 'Existing playlist item' and click on the file from the list, then on the next screen choose the FLAC codec, next screen leave encapsulation as raw, next screen choose the file to save to, but change the extension to .flac, then onto 'Finish'
- Open .flac file in Audacity (don't forget to add the LAME MP3 encoder to Audacity if you want to export to mp3), and there it is. Edit it if you wish then export to whatever sound file you wish.
[crarko adds: I think your milage may vary on this one, but I know it is useful to occasionally pull a sound snip from a video file.]

