You can control the nav volume (in a setup menu) through MMI..(somewhere in there - setup button while in the nav screen??) just turn the volume all the way down.

So you want to have it give you the blue route line and directions in the drivers display but no volume??

If it's because it's too loud you can control that, also if you are on the phone via the cars bluetooth, it somehow mutes the phone while giving directions, so your caller never hears the nav voice.