This may depend on what kind of alerts you want. Those can be very different things. I will try to discuss a few things.
Actual alarms could be as simple as triggering a external siren device like a Dome Siren. I have attempted to add this functionality to a Smartapp i have writen called ADT Tools . The idea being it monitors for triggered alarmed events and then triggers the siren/strobe, or flashes lights. It is also setup to stop the siren when the alarm is cleared.
I am fairly certain there are apps out there for triggering the door chime based on contact sensors this should be something completely outside of the ADT part, and just done by monitoring the contacts.
Does that cover what you ment by anouncements and such. Clarification on what that is exactly may help get to a good answer for all of the situations you are thinking of.