I'm sure you would have seen a video somewhere on the internet where you wanted the audio from- maybe a song or some other video. I found a website yesterday where you can add the URL of the video and it converts it for you. You can find it
here.
If you have firefox, you can download an add-on that creates a button on the video page so you can download directly. It's very convenient. If you go to the page mentioned above, there's a small tab saying "Firefox Addon" and you can download it from there.
image from the websitePlease note, I'm not getting anything for sharing this website with you, I just think it's useful. Hope you enjoy it!