You don't have to pay a service if you have the navigation maps. Not sure what you mean by Audio? It's not like OnStar (where a live person talks to you), but there is a computer voice that tells you where and when to turn. If you keep the car for a few years and live in a newly developed area you may need purchase a new dvd so that the maps are up to date, but there is no service fee.
How the nav works:
Your car is being tracked by satellite which then relays that signal to the car so that your location shows up on the map (maps are located on a dvd). So when you enter your destination to the nav system and you need to turn the "car" will prompt you to turn left or right. You route will be highlighted on the nav screen also.
Hope that helps!