Assuming Nokia have a licence to use mp3 from the Frauenhofer institute, if they are then sued for a patent not covered by those held by Frauenhofer (is this possible, or is the Frauenhofer patent and licence for the entire mp3 process?) they can sue Frauenhofer. Or probably more likely is that as so many large companies have licensed mp3, they just club together and bury/buy off the new patent holder.