If you’re a Debian user you may need to verify which repositories you’re using. Apparently one of the unofficial Debian multimedia depositories was using the domain debian-multimedia.org. Unfortunately the domain lapsed and is no longer pointing to valid Debian packages.
Not sure if you’re using it or not?
Simply run this command in a terminal:
grep debian-multimedia.org /etc/apt/sources.list /etc/apt/sources.list.d/*