Reading package lists... Done W: GPG error: http://debian-multimedia.org squeeze Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 07DC563D1F41B907

apt-get install debian-keyring
gpg --keyserver pgp.mit.edu --recv-keys 1F41B907
gpg --armor --export 1F41B907 | apt-key add -