Buster, re: #3- yes masks the most likely cause of garbled PA's... also, having heard the auto depressurisation PA on the ground a number of times, it's sufficiently loud in 'normal' circumstances but I doubt anyone would hear it very well what with people shouting, the step descent and the disturbed airflow....
If I recall correctly, one of the crew on the UAL 801 flight commented that she couldn't hear the crew member seated next to her, let alone anything else. (Different incident I know, but it probably gives an idea of just how much noise there might have been 'during the event')
Did your brother mention if it was more audible 'after' the descent (in level flight), maybe that will give an idication as to whether any improvement is needed....
Can anyone in the know tell me.... along with the auto announcement, does anything display on the pax screens aside from the 'fasten seatbelt' diagram (that is, if it doesn't, perhaps the steps for oxygen mask use should be displayed as well) I've never actually bothered to look at the screens when the announcement was playing on the ground...