Can i customize an electric horn on a car with a sound board and some buttons?

Car horn laws are pretty strict, and you cant modify them in any way that interferes with the intended purpose of warning other motorists. You can futz with the volume, within established Db parameters, but anything that significantly changes the tone of the horn or otherwise negates its use as a warning could be deemed an illegal modification

There is no way to amplify your car horn. Get a louder horn.

Anything that sounds unlike a car horn actually requires a permit according to most sound ordnances in cities and towns. 

Car horns are buzzers rather than speakers. You’ll need to add a weatherproof speaker, something that can survive where you put it.