Thanks for coming to the forums to look for a solution! Sure web converters are a dime a dozen, but they don’t always work and are constantly redirecting you to malicious web pages. I have found that you can use a piece of software from Wonderfox called HD Video Converter Factory Pro which offers a 100 downloads trial before you have to pay for it.
Once you have the program installed you can right click on any Facebook video then left click on show URL so you can copy it, then head over to HD video converter Factory Pro, click new download, paste and analyze the link, then select m4a audio and download away. Once you have the converted file you can then upload it to your mobile phone or even your google drive or alternative cloud storage if you have one to play on the go.
If you are happy with the software then you can decide on weather or not it is worth the value, they do offer a one time lifetime license.