If you copy the youtube URL of the video you want, paste it into the text field of the following site and it'll provide you a download link for audio only, from memory I think the URL has to be from a desktop version of the youtube page but it'll generate and download to your n9 all the same.

www.youtube-mp3.org

Good idea for an app or added feature for cutetube, I can't see how an app could avoid downloading the video itself to separate audio from video though, that may be a bit self defeating.

Edit: Oh yeah and you can use OVP instead of the default player to play videos in the background like cutetube, not so sure about using it directly from cutetube without making it the default player.