I understand those libraries come by default with IDEs, so its normal not to have them with our SED, right? How can I add them, or something?
IDEs very rarely ship with anything more than a handful of default and compiler/platform dependent libraries. I'm not aware of an IDE shipping with ffmpeg.
You won't get ffmpeg to compile with the Lite-C compiler, because it simply is not capable of that. You'll need a real compiler to get that going, so a plugin is what you will have to go for.