I'm aware the following might come across as mansplaining, but the specifics aren't important, or at least it can be saved for another thread. It's very difficult to say whether eight speakers is excessive as acoustics and issues around noise are a lot more than simply the number of speakers; consideration has to be given to volume, direction, placement of the speakers themselves, surfaces which the audio can bounce off or be absorbed by, as well as surrounding issues such as noise from trains or a nearby factory etc. It may be that a larger number of speakers were used deliberately so the volume from each individual speaker could be lower to still give the desired audibility across the station.
The audio signal itself (in this case, the train announcement) is also relevant - when we're talking amongst ourselves, we don't speak at a consistent volume, so there will be natural bits in our speech which are quieter (troughs) and bits which are louder (peaks). Pre-recorded audio will often have been processed after recording (or indeed possibly at the point of delivery by the PA system) to adjust the signal so that the peaks are made quieter, the troughs are made louder which gives the perception to the listener that the audio is 'louder' even though actually the volume is the same ('compression').
Additionally, the levels may be adjusted to increase or decrease at a certain audio frequency ('equalisation'), for instance a radio presenter may have their system set up to increase the audio signal level at the lower frequencies to give them a nice bass-ey radio voice whereas a train announcement might actually want to reduce lower frequencies and amplify the higher ones because the train announcer needs to be audible over loud, bass-ey diesel engines which may be sitting in the platform. The problem with this then is that denser materials (and train stations will often have concrete/tarmac etc platforms, buildings, walls etc) reflect higher frequencies better than lower ones so the audio may echo, further adding to the noise problem.
Ultimately there's an expectation that whoever you live next to, whether that be a nightclub or a railway station, they'll do their bit to minimise noise issues for their neighbours. Indeed, if Northern conform to ISO14001 they should be using noise complaints as a KPI and have procedures in place to investigate these in the same way they would if you were to complain that their trains were discharging oil into your garden or somesuch. Whilst living next door to a railway station you should expect a certain degree of railway related noise it's not unreasonable to challenge excessive noise where even perhaps simple mitigations like repositioning speakers etc may make a difference. As has been noted upthread, it's also certainly possible to install systems which measure the ambient noise and alter the volume of the announcements based on that.
