If you use Apple Maps for iPhone, odds are you use the voice navigation feature as well. Depending on how you use it, the default volume may be too low to use without an external speaker or too loud if you're using your car speakers.
You have the option to adjust this default setting whenever you'd like. Here's how:
Launch the Settings app from the Home screen of your iPhone or iPad.
Scroll down a ways and tap on Maps.
Here you'll see an option for Navigation Voice Volume. Here you can adjust the loudness of the default volume Maps will use.
If at anytime you'd like to override the volume, you can of course use the volume controls to do so.