Right now, it is more accessible to downloading video, mp3, program or else using Download Manager because that software can directly download that thing. If you type "youtube downloader" in the search engine, you can get a bunch of the list that you can use to download a video from Youtube, a website downloader or software that will help you to download.
Correction mate , if i understand it right if you are referring to mp3 that means Music and if you will download Video then you must use mp4 instead?
sorry if i understand it wrong but i am not really an internet video music guy lol.
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You can modify the keyword term to "youtube convert mp3," or anything you want and you will get tons of websites to convert the video from Youtube into mp3. You can change the keyword as what you want to search.
Using Download Manage can also convert video to mp3, if I am not mistaken.