Nyssa, OnStar is a cellular phone built into your car basically. It's tied into your cars communication system so it can send commands to other modules on the car to do things like remotely honk the horn or unlock the doors under remote control. It also has a GPS antenna so that someone can remotely identify your vehicle's location.
They can help you get where you are going, tell you where you are or track your vehicle if it's stolen. It's under the control of the OnStar center, if you have a problem you call an 800 number and as a subscriber they help you in time of need.
Visit
www.onstar.com for more info. As for whether or not it works outside the US... I don't know. I think other countries have variants of the service, or they may be tied into the same thing. I am sure the equipment or setup may vary based on the country you are in.
gort, get me the last 8 digits of your VIN. There are lots of variants in that system based on what options you have. It sounds like you probably just lost a door speaker but if you are handy with a voltmeter I can tell you how to check it. If you remove the door panel and disconnect the suspect speaker you can check for ac voltage at the connection. If it reads right you probably just need a speaker. With some wires and a AA battery you can check the speaker as well. Let me know.