Why does sudo ask for a password?

USER@notebook:~/foo/notebook$ sudo /usr/bin/mplayer /home/USER/foo/notebook/something.mp3
[sudo] password for USER: 
USER@notebook:~/foo/notebook$ 

USER ALL=(ALL) NOPASSWD: /usr/bin/mplayer /home/USER/foo/notebook/something.mp3

Question: sudo didn't ask for a password. Now it asks.. Why? The sudo package and the sudoers file wasn't updated, the USER's pwd isn't expired.

Playing an mp3 via ssh and sudo is needed because it doesn't play the sound without it, strange..

I'm using Ubuntu 12.04.