From the FAQ:

http://www.brickboard.com/FAQ/700-900/RadioAntenna.htm#RadioCodeLost

Radio Code Lost. [Problem: Lost radio security code.] [Response: Bob] Codes are available from your local Volvo dealer if you have the car's VIN or the radio serial number. When a dealer replaces a radio, they are supposed to update the Volvo data base with the new radio code, but most seldom do this. If you don't have the VIN of the donor car, you must supply the dealer with radio model and serial numbers. Pull the radio and look for the serial and model number pasted on top. To remove it, follow this link.They may ask for proof of ownership before they give you the code. They should do it for free. While they are on the computer, ask them also to get your key code #'s if you don't know those. They could come in handy some day.