You need to read your manual and learn to use use idrive. When you are on the Nav screen, use the Idrive control knob to scroll up to the voice command icon. I believe it is the top one or 2nd one from the top (has a person talking on it). Once it is highlighted, push in the idrive selector and a check mark will show up. Voice command is now active.
Read your manual if you want to learn how to do the other volume settings in Idrive, but it is all there. Hope that helps.