I know of a couple of methods to achieve what you want, but first, have you considered that what you are downloading may actually be an html file and not the video that you want?
One simple method would be to visit this site http://www.sothinkmedia.com/guide/download-video.htm and download a plug-in for Firefox that will download video as flv files. The same site has links to software that will convert flv to mpg or whatever else. Then you can use Switch to make the final conversion to mp3.
Alternatively you can use the procedure given below based upon - http://googlesystem.blogspot.com/2008/04/d...-mp4-files.html The terminology used below is for Windows machines but there is equivalent functionality for MAC
http://www.youtube.com/get_video?video_id=...TURE&fmt=18
is the model URL for ripping youtube videos
Example:
Louis Armstrong and Velma Middleton - St. Louis Blues http://www.youtube.com/watch?v=9nwHdvxEDaU
Load the video and then "View source" - look for video_id parameter and t (SIGNATURE) parameter.
Searching for the First reference to video_id found the following
var swfArgs = {"vq": null, "video_id": "9nwHdvxEDaU", "l": 375, "sk": "LkEVTlk0wNbMtGMQNje4LE8YKiNnXBHYC",
"fmt_map": "", "t": "OEgsToPDskJ9GP6D-8e-GudR3QuBTFlr", "hl": "en", "plid": "AARV2PhH6DBLNE4YAAAAqAAwABg"};
Here ID is 9nwHdvxEDaU and t (SIGNATURE) is OEgsToPDskJ9GP6D-8e-GudR3QuBTFlr
Create a "shortcut" to
http://www.youtube.com/get_video?video_id=...TURE&fmt=18
and substitute 9nwHdvxEDaU for ID and OEgsToPDskJ9GP6D-8e-GudR3QuBTFlr for SIGNATURE
click on the shortcut and save the video as an MP4 file under a name of your choosing. The use Switch to convert the mp4 to an mp3.
Good luck.