That is why it is called "Infotainment System". "Information/Entertainment System". Two systems merged together with ONE power supply. Have your tried "pause" instead of mute?
I may be wrong on this (haven't tested it), but I believe there are separate volume controls for navigation and radio, so you should be able to just turn the radio down to zero and the navigation voice will still play. Ta Da!My personal solution to this is to play a short youtube video on my iPhone. I have one bookmarked that ends in 15 seconds. Once the video ends, the bluetooth connection is paused and there is no further autoplay. I do this trick whenever using the Nav on a long trip. If I remember before leaving the house, I play the video there first so it is already queued up. Then when I get in my '14 Volt and turn on the Infotainment system set to bluetooth (previously paired to my phone), the same video autoplays again. But once the video ends ... silence (except for the directions of the Nav system). Ta Da!