Thank you for the reply. No PA system. The project is to produce horn sounds rather than my air horn. The power input is 24v so a buck converter is required for the Adafruit sound card. I will trigger the sound files with buttons. From the sound card to an amp (mono100w?) to a speaker. I can pull the items from Amazon but wanted to source from Oz if possible.
Ah ok. So we want this to be pretty loud. If It were me I would get one of those boom boxes for $40. They have speakers and surprisingly beefy amps built in.
That said, CE might have some ideas from there stock that can produce that much bang.
For the converter, something with a regulated 5V output would be ideal. Either of these two would work, depending on whether you would prefer a bit of headroom about 24V for the input:
As for an amplifier, one of these options may work for you although if you can get your hands on the exact wattage of your speaker that would be ideal.