This is definitely true - and I think puts the finger on why this subject is quite difficult to understand.
Let me try something else. Say, you are a political activist. You are committed to a certain way of understanding the world, a certain frame of belief, a certain set of convictions. You want to impose these convictions, transmit them, put them into wider circulation. How do you do this?
Immediately, a certain number of material questions come into play. First of all, there is the issue of how adequate your understanding of the world is to present circumstances. Or maybe, not adequate, but persuasive. Certain frameworks will be resisted by the world, for various reasons. I don't think I would have that much success if I tried to convince people that the ultimate political imperative of our present moment is to, say, overthrow the lizard people.
So here is where the wider points about your socio-cultural background come into play. The socio-cultural allows for certain possibilities, and blocks certain others. It is easier to transmit certain messages then others - because of the shape of the message, because of the shape of the receiver, and so on. Someone like Alain Badiou seems to me perhaps to deny this mediological aspect, insofar as he says that politics is ultimately a matter of axioms, of insisting on axioms.
One point: Certain radicalisms (Marxism, extreme rightisms) argue that their traditions and perspectives are systematically suppressed by prevailing extant systems (capitalism, liberalism). This is worth considering, both on a rhetorical level (consider the psychological appeal of the discourse that "they don't want you to hear") as well as on a practical level - it is true that there is a certain prevailing way of understanding information instilled by the media. Though in the end this might be more formal then ideological - less important, ultimately, I think, then what you want to say, is how you want to say it.
This is kind of scattershot, but maybe someone will be able to shoot these arrows off somewhere.