If the the voice command is too quiet during a navigation, then you can change this:
- Swipe right to the Main menu. Then go to Settings > Sound > Volume.
- Press on the volume control key (hardware-key) of your device until the desired volume is reached.
The audio playback of the navigation voice command is always set as media.
Should you have further issues with the audio playback, then we recommend checking the settings of your device.
For further information visit the Apple homepage: About the buttons and switches on your iPhone, iPad, or iPod touch