Gasmeter, I wondered in my original answer if the sounders were loop-powered. As Graeme stated above, in the basic operation function of these sort of devices, the system needs to turn them on individually and turn them off individually.
Particularly on multi-loop systems and/or heavily loaded loops it can seem a long time after sending the control signal (e.g. sounder silence) before the last sounder reacts.
However, a minute seems far too long unless there is a further issue with the way the control panel sends it control signals.
It is good that you are going to check this 'problem' out yourself because you will be able to experience and take note of all the relevant information required to be able to properly assess this problem. (I have personal experience of supposed 'faults' being wildly exaggerated. Some reported faults even defied the laws of physics"!)
Again, as Graham mentioned, some sounders and control equipment have a facility known as group addressing that will be able to turn on and turn off all loop powered sounders to one control panel signal. This might be what you need to resolve the 'problem'