I can't adjust the Google Map Navigation volume, even when I follow the steps given by edubb256.
And the funny thing is... I'm pretty sure I could adjust this app's volume before (week 1 of S8 ownership). I could also set it to play always through the phone speaker instead of my car's bluetooth stereo (which I want to reserve for phone calls, music, audiobooks, etc). This feature used to be available within Google Maps, but it was removed, and now it appears to have been removed from Samsung's SoundAssistant app. WTF