Colin,
There are several different sounding ahooga type horns. The 1928-1931 Fords all came with one standard from the Ford factory. They sound like the one at:
http://www.ahooga.com/ahooga_wav.shtml and are powered by a six volt battery. If you keep the horn button depresses they will continue to honk and can be honked longer than the one at that link.
The earlier Model T Fords and many other early cars sometimes added an accessory Klaxon / ahooga horn that was hand powered. I.e. you push a plunger that spins a wheel that vibrates the horn's diaphragm. They sound very similar to the ahooga at the site above. They can only do a short ahooga and cannot be held for extra seconds as the plunger goes down (or in) and make the noise while the plunger is moving. Below is picture of a Stewart Warning Signal that is one of those aftermarket Klaxon/ahooga horns. It is a rather loud horn in a small room but would probably need amplification for an auditorium.
If you would like to borrow it, you may. Hopefully you will find one closer to you but if not we could mail it or if you know of someone headed that way we could arrange to have them take it to you.
Warm Regards,
Hap Tucker
803-469-7792
Sumter SC