On the subject of rapid descend, the reason why it is like that, is to get down to breathable air asap, regardless whether the masks are working or not. The oxygen usually lasts 12-15 mins, so we need to get on the ground asap. It's procedure.
On the subject of an automated PA during a decompression, the airline I work for, on the 737, we do have that PA system on board. It also has a recorded announcement for ditching and emergency landing. It is loud, very loud and very clear. It tells pax what's happening and what to do. I may be presumptious, but I would think other airlines have this system as well, but I may be wrong.
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