(2004 Lincoln Navigator) How to make it beep when I lock the doors?

It should only beep on second press so make sure your sending the lock/unlock signal twice. I'm a little surprised one would make it beep and the other wouldn't but if its not working like it should you can try this to activate/deactivate horn chip , press and hold the lock and unlock buttons simultaneously for 2-3 seconds. You should see the hazard lights flash. Hit the lock button once. Now the horn should honk if activating or not honk if deactivating horn chirp.