I think it does not help that the quality, volume and clarity of announcements vary so hugely between trains and operators. Two (somewhat humourous) examples below:
One day (think it was the night after the Hook and Winchfield landslide) I was at Waterloo, waiting on the first carriage of the 2108(?) local stopping service to Guildford. The driver was just getting on and having a chat with the commuters, and was in the process of opening the door to the driver cabin when the fire alarm suddenly sounded. The driver went back out of the driver cabin and announced loudly to the cabin, "You're under me, stay on the train. I'll see if I can get you out of here." However, as he had not used the train PA system, nobody in the rear carriages heard anything at all, and as the driver went off to check with control, a considerable part of the other three carriages emptied out and headed for the barriers as platform staff were directing people still on the platforms to do. In the end, it transpired to be a fire alarm drill, and the driver came back in and pulled out about five or six minutes late. I'm fairly certain however, given the time elapsed between the fire alarm stopping and the train pulling out, that not everyone emptying out in the confusion managed to get back on the train. (There were also several who could not tap in at the barriers as they were opened during the fire alarm, but luckily the guard was understanding.)
Another time I was on an EMR service out of Bedford to Sheffield. Announcements were not frequent, but the audio in the cabin was very inconsistent, leading to about 90% of the message being blocked off, but the remaining 10% being played clearly at a loud volume. It did not help that the driver had quite a strong accent. Just before the call at Kettering, the driver made a manual announcement which I assume was intended to be along the lines of "The next stop is Kettering. This train will be calling at Kettering, Leicester, Sileby, Loughborough, etc. etc" but due to the variable audio, what we heard was "Next stop... Loughborough". Upon hearing this, a man jumped up in panic, hurriedly gathered up his luggage and belongings just in time as the train stopped, and shot off the train. Having dropped his coat in the hurry, he spent some time putting it back on, then started to walk down the platform. About 15 seconds before the train was due to depart, he apparently finally noticed which station he was at, visibly exclaimed in terror and rushed for the train doors, which closed in his face. The train pulled out of the station leisurely, leaving the poor man staring at us forlornly.