The Secret Ops full starter package (including speech) must be installed
Unknown must be installed in the exact same folder than Secret Ops
There can be only one Secret Ops folder, since it's referenced in the registry (and it must be the folder you originally installed Secret Ops to). Don't think of making a copy of the SO folder and installing Unknown Enemy there, because it won't work. Don't worry, having Unknown Enemy being installed in the same folder than Secret Ops won't alter the original game, so you can play still play both SO and UE.