1. Press H. (location of H for keyboard and gamepad below)
2. If it doesn't work press H again.
3. Might wanna try holding H instead.
4. Keep pressing, press faster, harder, better.
5. You should be hearing some noises by this time.
6. If it still doesn't work go into options and turn down your audio settings, ambient especially.
7. Back into the game and hoooonk!
8. Now you know how to work that phat horn!