I thought it was to mute the NAV if you were on the phone? Otherwise I agree I have not found any setting that I can adjust to reduce or mute any audio source when NAV announcements are made. It's totally different in my Audi and if the radio is on or if music is playing from one of he onboard sources it's muted when there is a NAV instruction. I can set the level of mute in the MMI system. But the VW system is not the same.