Believe it or not, sometimes standard outlets can be used for different voltages, despite being a very rare occurrence. For example, the Illinois Railway Museum’s Milwaukee Road X-5000 Dynamometer car has standard outlets that supply 32V. These outlets are clearly marked as such and this 24VAC Federal 350 operates off this power.
Another scenario could be that that the outlets powering these horns were wired in series. With the voltage being distributed equally among the horns, 9 horns could be connected in series to a power source supplying 110-120 Volts, allowing for a roughly 10% tolerance in the 12V feed to the horns. The plugs, rather than hard wiring, could have been used for ease of replacement. This is an example of such a practice at my dad’s work building:
So, in reality, there are several possibilites as to why they are fitted with standard 12VAC plugs