I noticed today that the volume control knob on the dash is actually dual purpose.

I was listening to the stereo when the nav directions chimed in. I attempted to turn up the stereo volume, but instead the Nav voice kept getting louder.

If you adjust the knob while the Navigation voice is speaking it will control the Nav volume, and of course while the stereo is playing it will control the stereo volume.

__________________
2013 X5 35i M Sport