Assume I have the id (The numbers and signs in the video's hyperlink after "http://www.youtube.com/watch?v=") of a Youtube video. How can I download the audio stream of the file to an sd-card as an mp3 with android code? (I wrote an application for 2.x but this part is missing).
Look at how some other YouTube scrapers do it, e.g. youtube-dl. Then recreate their algorithm in Java for Android. Then keep spending time trying to stay up-to-date with the capricious changes YouTube will make to their site every now and then.