Video ID:
The part of url after v=
(http://www.youtube.com/watch?v=n9AcG0glVu4)

Or you can go directly to url http://yt.akmu.org/?v=n9AcG0glVu4

For audio only http://yt.akmu.org/?v=n9AcG0glVu4&audio=on

Files will be stored ~24 hours

Only audio: