[top ramen]

While bored and going through the navi today, I believe I've found the way to increase the navigation turn by turn voice permanently without having to increase the volume when the voice comes on while driving. I did this while my car was parked and didn't input any address.

Step 1: go into navigation screen with the maps.
Step 2: move the knob on the idrive to the left and scroll until you get to the speaker or voice option
Step 3: unclick and click until you hear the female voice. When you hear the female voice, quickly increase the volume on the steering wheel or the center console and you would hear her voice increase. If you like it louder or lower just repeat step 2 and 3

I took the car for a drive while inputting an address and having the music up pretty loud and I could hear the directions clearly.

Hope this helps or let me know if this works for you guys or not.