[demick]

I think if you just double press the lock button, the horn will chirp.

Another way (which is what I did), is to fool the car into thinking the armrest storage bin lid is not latched. This will trigger an audible chirp when locking the car. Test it out by not closing the lid completely and locking the car. If you like it, then you can simply disconnect this switch so that it will do it all the time (access by removing the ashtray).

Since the armrest lid notification is completely unimportant anyway, you can actually get a useful function out of disabling it.