Possibly because the previous owner removed the speaker or installed a different alarm system?
Possibly the horn is just broken? Call/contact the dealership and ask them your question...
Edit: Try this
link. The second post says that if you haven't tried activating it via the manual (and I know you can probably find this procedure from a simple google search) then it's shorted or otherwise broken. It should sit right on the grill and again, if you do a google search for a parts diagram for the front end of your car, you should be able to find a part number/find where it's located on your car.
Hope this helps!