You could copy and paste the YouTube link to the Download box on

savefrom.net, or simply add ss before the YouTube link to see all available resolutions for a given video, from there you will get to see an option to download the video in Audio MP4 format. Do the same.

Or you could paste the YouTube video link to the Download box on keepvid.com and download the video in M4A format which will give you the required audio file.

Good Luck !