OS 15 GPS Navigation voice too low or too loud
I’ve found the answer to adjusting the GPS voice volume on IOS 15. The volume settings on maps has been removed. Go to Settings > Siri & Search > Siri Voice. Select the voice you want to use and while it is speaking use your volume adjustment buttons to turn it up or down. It seems to have a seperate volume control that can only be adjusted while it is speaking. This worked for me, hope it helps others.