Take a look at each door after you have pressed the lock button and see which one hasnt gone down. The horn basically signals that either the actuator has gone (in which case the door cant lock) or a front door is ajar whilst trying to lock it (also meaning the door can't lock). Rear doors will lock whilst open, however I don't know if the horn will signal when an actuator goes in the rear door. Anyway, best chance to find out whats going on is to look at each lock on each door when you lock it, the lock that doesn't go down is your faulty one.
Ben