Several issues occur with the door locks on the Volvo C70 and most are because of the door lock assembly. What happens is that one door will not lock while the other one does. In other cases you will lock the doors and then one of the doors will immediately unlock.
The problem is that the door lock assembly is defective and will need replacement. Replacement of this assembly on the C70 is very difficult and is not recommended by a novice. Getting the door and window aligned properly so that you don't get air or water leaks is very, very difficult.