As mentioned by others, the radio code will often be found recorded in the vehicle manuals. For a used radio it will typically be found hand written on the radio case. If you can't find the code or it doesn't work then you can get Volvo to look it up for you, either by VIN (which only works if it's the original radio) or by the serial number on top of the radio (the proper method, but does require sliding out the radio -see the FAQ on how to do this). Give this to your local Volvo service center and they will fax/phone it to Volvo Cars HQ. Technically they're supposed to establish ownership, but showing up in person usually suffices. There should be no charge for this service, but it may take a day or so to get a reply. For procedures on entering the code, if you don't have your owners manual then you can go to the Volvo Cars of North America website and download a copy for your vehicle from the Volvo Community section. The Volvo Cars website is perpetually being changed, so you may have to poke around for the owners manual downloads starting at the home page.
--
Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now