Good Day!
I understand what you're trying to say! Maybe this will work:
* Click the TAB key on your keyboard
* Go to the heading `sound
* and lastly, Check `horn sounds
This should do the magic job. If not, try uninstalling TruckersMP along with Euro Truck Simulator 2 from the steam library, and then reinstall it. This is usually the most special trick that works the most ?
(or maybe try the things said above by people. It's actually much easier ? )