Thank you for posting a reply, Randux.
I tried your solution but it didn't work...... :
Code:
# gpg --import ./debian.gpg
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
Well, "downloading the key and importing it to your keyring and running again" is a solution on the web page that I mentioned before. I followed ALL intructions on the web page.
As for
www.debian-mutimedia.org, I could download/install keyring package from the web page.
However, I can find neither keyring package nor the gpg key to get it working on
www.debian-unofficial.org. (I could download the key, but didn't work.)