Can noise from the passenger cabin be picked up by the cockpit voice recorder?
It depends a bit. One of the microphones for the cockpit voice recorder is an "area microphone," which is designed to record whatever someone standing in the cockpit would hear (the others record from crew headsets and possibly the PA system). This area mic is continuously recording; it does not just turn on when there's more than some threshold of noise. If the noise in the cabin is loud enough to have been heard in the cockpit, it will be heard on that mic. If not, it won't.