I found this explanation on another forum and it worked on my Mercedes-Benz.
“You change the CAR'S VOLUME CONTROL during a moment when siri is giving audio voice guidance for map directions.
You must do this during a moment when siri is actually speaking the directions
What changes is the car's volume for siri's spoken navigation directions.
The volume the car subsequently uses for iphone music or audiobooks, for example, is NOT changed.
Just to be clear: CarPlay knows when its speaking navigation guidance instructions and when its playing music, and siri automatically uses the volume level you last chose for each. If its playing music and voice guidance instructions at the same time, siri interprets a change in the cars volume control (at that moment) to be intended specifically for voice guidance”