This directly violates the Youtube API terms of service. https://developers.google.com/youtube/terms

Under Prohibitions: "8. separate, isolate, or modify the audio or video components of any YouTube audiovisual content made available through the YouTube API;

9. promote separately the audio or video components of any YouTube audiovisual content made available through the YouTube API;

10. access any portion of any YouTube audiovisual content by any means other than use of a YouTube player or other video player expressly authorized by YouTube;

11. store copies of YouTube audiovisual content;"