I prefer to convert mp3 which allows users to convert YouTube videos to mp3 format in seconds. Here you can download YouTube video to mp3, mp4, Avi, WAV formats Free mp3 converter tool.
Well, usually there is some software and website specially designed for this purpose.
You can use it to directly convert YouTube videos to MP3 format.
YouTube2MP3 to download Videos with One-Click.
There are many of the similar web sites. I tried a few of them and found the site: https://musicdownload.zone is pretty cool. There is no ads and clean. It is fast and easy to use.