If you do not need to have speaking instructions in Apple Maps navigation system on iPhone then guide below will help you to know how you can adjust navigation voice volume in Apple Maps app.
Disable Voice Navigation in iOS Maps
Step-1
Tap on “Settings” and then tap “Maps”
Step-2
Simply select “No Voice” option under NAVIGATION VOICE VOLUME and thats it
See also:
How to Change Default Volume for Voice Navigation in Maps for iPhone