check out this youtube mp3 converter - very easy just look up video on facebook and then convert to mp3
I personally use this youtube converter to convert and download youtube videos and it has given me amazing results so far it also to download facebook videos
You are stepping into VERY scary territory here. Don't ask questions on the web like this if that is your actual name and photo in your profile. Just friendly advice to keep you safe/jail and fine free. There are no *cough* ways to download any old song for free unless the artist posts it for free. Soundcloud might be a good bet if you are just looking for any old free music tracks, a lot of underground artists who give away free music to try and get their stuff out use Soundcloud.