Alternately, you can download the Youtube video and rip the audio yourself.
Using an extension like
DownloadHelper will let you grab Youtube or NicoNico videos. Make sure, to get the best possible audio and video, you append
&fmt=22 to the end of any YouTube vids, which will show them at their upload quality instead of the stepped down, quick-stream quality they usually show at.
Once you have the vid, using a program like
FLV-Extract (which is a dirt simple, drag and drop audio extractor for .flv filetypes) or something more robust like
AOA Audio Extractor which can strip audio from a wealth of video types with just a few clicks.
Both of the latter programs are freeware, and are great tools for getting your hands on audio tracks.