The Maps app in iOS 6 and later offers turn-by-turn directions using Siri’s voice. But most iPhone and iPad device owners don’t know how to adjust the volume of that voice. It’s not where you’d expect.

Fire up the Settings up, and scroll down until you find Maps. There, you’ll see controls for disabling the app’s voice, or making its volume Low, Medium, or Loud.

Source: Mac OSX Hints