More likely Radar and IR stealth than silenced. It was the trip in from Afghanistan they concealed. By the time they were over Abbottabad it was too late to do anything about it.
It could be too late to prevent the strike, but it wouldn't be too late to shot down the surviving choppers.
Of course, in that situation, the F-22s would have protected them.
Could they even make it to Afghanistan? I was under the impression that helicopters don't exactly have a whole lot of range