I use a download manager called Jdownloader...its free
This GUI allows you to download almost anything..
What I do is copy the YouTube URL addy and Jdownload grabs that link and will automatically decrypt the URL into different files and that will include Mp3 Mp4 etc.. you will see it in the window of JD..
Try it and see...
Also there are other programs out there that convert but I find Jdownloader is awesome for this..