If the the voice command is too quiet during a navigation, then you can change this:

  1. Tap on the left side at the bottom of the screen on the route indicator.

  2. 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.