Summary says it all. Would be good information to know if somebody is choosing non-free (as on speech) software.
All of them are non-free, as per the definitions from the FSF. In some countries (like the US), those plugins require payment of a patent license (that includes the mp3 decoder, and the mpeg demuxer). So they can never be considered free, and the availability of sources doesn't change that. On the other hand, the Fluendo site should have more prominent links to the sources for the plugins that have some available. I filed this as: https://core.fluendo.com/gstreamer/trac/ticket/77