Do ordinary Muslims have any responsibility to speak out against extremists?
If so, what form should that responsibility take?
If so, what impact would their statements have?
Even if we assume that such a platform were readily available, it's fair to ask, who would listen? The extremists and their supporters don't care if they're being denounced. Those suffering under their oppression aren't helped or empowered by a denunciation. Those calling for the denunciations are likely to either ignore the expression, assuming it even comes to their attention, or regard it as inadequate because it doesn't actually change anything.
Ordinary people are not ordinarily tasked with separating their own beliefs from those of extremists. Even in high profile contexts, we don't expect people to distinguish their mainstream religious beliefs from those of extremists within their faith. For example, Mitt Romney was not asked to denounce the FLDS based upon its claim that it represents the true face of Mormonism. But what if he had been asked to do so, or had voluntarily condemned the FLDS? For those in FLDS communities, not one thing would have changed. For any who might to point to the FLDS as representing the truth about Mormonism, the statement would mean nothing.
At the end of the day, the calls for denunciation seem to come primarily from two corners. First, from people who are using the absence of public condemnations, real or imagined, as a basis to perpetuate their own prejudices, and second, from commentators and demagogues who benefit -- who seek fame, attention, and profit -- from engaging in anti-Islamic rhetoric.