zypper ar http://ftp.gwdg.de/pub/linux/packman/suse/openSUSE_12.3 packmanThe package install may prompt you to allow a vendor change (from the 'official' openSUSE repository to the Packman repository). This vendor change is desired so it should be allowed. Once the packages are installed GNOME Video player should play WMV files which use ASF multiplexing without further complaint.
zypper in gstreamer-plugins-bad-orig-addon gstreamer-plugins-libav
Add the Packman repository and install the required packages.
Aside: If you don't want to be pestered about package vendor changes in the future you can edit /etc/zypp/zypp.conf and set "solver.allowVendorChange = true". But don't do that unless you know what that means.