update: probably useless compared to youtube-dl
if you wanted to download audio from youtube you just head to solutions like keepvid that re-encodes youtube audio for you, off course this is a lossy operation.
I've been digging to find a way to just rip youtube audio as it is and found a basic way of doing that.
so
vlc > media > convert/save > network
enter youtube link in addition to &fmt=##
where ## is the desired codec from the table below (available on wikipedia/youtube)
convert > create profile with similar settings > browse for a new .m4a file (for aac) > convert/save > overwrite
it will then begin loading.
i think it works only with vlc versions >=2.2.2