The radio code can be anything from 0001 to 9999 and is cross-linked with the car's VIN into which it was fitted during production...
I know - found apps which takes the serial number and vin combination and spits out about four possible codes.
As in example: http://www.radio-code.com/Volk-Pay-Page-radio-code.htm
Although, the above is a paid service - some digging and you can find little apps to do it.