I'm not sure if I have understood your query and exactly what you are asking. But I'll answer what I think you are asking!
The pre-determined messages played over public address loudspeakers are normally part of the public address control and amplification system. In the very old days messages were stored on magnetic tape, but these days they are stored digitally in electronic circuitry. The playing of any message is normally automatically initiated by receiving a 'start' condition from whatever source the particular message relates to, for example, fire alarm control panel.
Recently, it has become common to use self-contained alarm warning sounders with a digital electronic message(s) stored in each individual sounder. These operate by connecting a power source to the device. No Public address amplifier or loudspeakers are required for this sort of system.
Whilst manufacturers of message modules for PA systems and self-contained voice evacuation sounders normally have a range of standard emergency and evacuation messages pre-recorded, they also normally provide a service where custom messages can also be provided.
Does the above answer your question?
