I've got Ant Video Downloader installed on my machines but I've also got this:
https://addons.mozilla.org/en-GB/firefox/addon/easy-youtube-video-download/
If you install that extension you'll find that there's a 'Download As' button underneath the video on all Youtube pages. That allows you to choose the format which you'll use to download the file, including 'MP3'.
If you want to extract the audio from an
existing video file on your computer, there are countless file conversion programs which can do the job, including Format Factory:
http://www.pcfreetime.com/
However if you've got VLC you can do it from there (which is what I'd do):
Open VLC
Go to Media > Convert/Save
Click 'Add'
Navigate to the location of your video file and select it.
Click 'Convert/Save'
Select 'Audio (MP3)'
Click the 'Browse' button to choose a dstination for your MP3 file
Check that 'Save as type' is set to 'Containers (.mp3)'
Click 'Start'