This patch gives all vehicles with horns a common configuration. This may be used to add more horns.
A handtool is also present.
This costs $380 and is located in the shop under "handtoolsMisc."
The typical setup costs $100, and it may be added to hand tools or cars using XML.
After that, you may add more sounds or your own rates.
Both the car and the hand tool include buttons that you may push to choose between the sounds that are available.

Thanks to nilBrain