A properly functioning Boxster alarm does not honk when it's armed.
The blinking red light on the dash - and the doors locking - are your indication that your Boxster alarm is armed.
If you'd like it to honk, remove the factory stereo and remove the tape that prevents the microswitch from grounding against the radio chassis (
http://www.987forum.com/forums/showpost.php?p=102764&postcount=11).
You can also try arming the alarm with the center console open to ensure your horn is working as expected - I believe this will also cause the alarm to honk when armed.
My understanding is that you can clip and ground the lead going to the microswitch to ensure that the alarm chirps when you arm it. Of course, the down side is that the alarm system will no longer be capable of detecting if the stereo is removed, so your alarm would not sound should a thief with very poor taste in electronics manage to remove the crappy factory stereo without othserwise tripping the alarm.
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