unfortunately, it's a complete violation of the youtube api terms as far as i can tell:

these are all prohibited:

separate, isolate, or modify the audio or video components of any YouTube audiovisual content made available through the YouTube API;

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

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;

store copies of YouTube audiovisual content;

https://developers.google.com/youtube/terms

i dont work for google or youtube or anything, but it seems that it should be common sense to read the terms of an api before using it.