Have you considered any apps already? I recommend to have a look at this free online video converter. Make just two simple steps to get your mp3 from YouTube. Copy-paste the URL of the video, click the “Convert” button. Flv2mp3 has a wide range of alternative formats to choose from: WMA lossy, Apple's AAC for Mac, DivX, mp3. It is pretty fast and handy. I`ve been using it for a couple of months already and it works pretty well for me. Cheers

Hi,

try 4vid, is is free and extremely powerful. It let’s you download any video from YouTube and many other platforms and websites. Hope it helps

How do I download YouTube videos on Android?