This mod adds a standard configuration to every vehicle with a horn. You can use this to add additional horns.
There is also a handtool.
This can be found in the store under "handtoolsMisc" and costs 380$.

The standard configuration costs $100, you can also add this configuration to vehicles or hand tools via XML.
Then you can add your own prices or additional sounds.

You can switch between the available sounds by pressing a button on both the vehicle and the hand tool.