If you are user of voice navigation feature of Apple Maps on iPhone, then you observe its default volume is too low. So, guide below will let you know how you can change default volume for voice navigation in Maps for iPhone.
Change Default Volume for Voice Navigation in Maps for iPhone
Tap on “Settings” app from Springboard of your iDevice
Tap on “Maps”
Under “Navigation Voice Volume” you can adjust the loudness of volume for Maps on iPhone