I use it for all vehicles, the vanilla ones and also all modded ones. You only need to add them to the vehicles.xml in the horn modlet, f.e. like this
<set xpath="/vehicles/vehicle[@name='manux_vehicleMonoHog']/property[@name='hornSound']/@value">khelldonBikeBell</set>
<set xpath="/vehicles/vehicle[@name='vehicleGuppyBoat2']/property[@name='hornSound']/@value">khelldonTruckHorn</set>
<set xpath="/vehicles/vehicle[@name='vehicleDirtBike']/property[@name='hornSound']/@value">khelldonMotorcycleHorn</set>
etc. I also renamed the modlet to XKhelldon-GetHorny to make sure it loads AFTER every vehicle modlet, so the new sound applies properly.