The easiest way is using your computer's browser. There are several sites that do this for free.
– Snappea – Flvto – Savefrom.net – Y2mate
– 2Conv – H2 Converter – video-to-mp3-converter – ytmp3
All sites work in a similar way. And you can download it in a matter of seconds.
The websites indicated can also be used in the mobile browser. They will work the same way.
To download just copy the video link and paste in the indicated place on the site. Then choose the MP3 option and then download.