I'm in favor of human moral development through means such as free speech. But "a sound fundamental philosophical justification" doesn't appear to have any meaning, or rather can mean anything to anyone. It can envelop humanity's destruction as easily as its flourishing, depending on where your identity lies.

I'd like to be able to deploy "natural" in defense of free speech, but I've found no substance to it. No premise beyond a personal value judgement, which I happen to share. So I default to appeals to utility.