On the title screen, hold [Left Shift], then use the Left Mouse Button to select the round speaker.
Tesla tanks (Counterstrike mission disc)
Begin a game in skirmish or multi-player mode. Choose to be France and achieve a tech level of 7 or greater. Then, hold [Shift] and click on the radar jammer icon.
Free troops (Counterstrike and The Aftermath mission discs)
Build a missile silo and wait until a missile is ready to be launched. Then, sell the silo and immediately launch the missile. When timed correctly, the sale will be aborted and men will exit the silo.