You probably are looking for a way to download Youtube video (perhaps music video) and then wish that the download files are in mp3 format. Actually, this can only be done by conversion. You may use offline software that converts the file immediately after you download. You can also use online converter and downloader. This process of downloading and converting will take a lot of time. Surely you need to wait.
One of the many online web application that you can use to convert video into mp3 and then download it is Snipmp3.com. Through this website you can do the two tasks easier. What you need to do is to paste the link and then wait for the file to be downloaded. That's it. Be patient waiting for it to be downloaded.