Navigation Commands were load and clear initially. For the last month or so I can no longer hear them. The music pauses during a command but no navigation sound. Nexus 6P
During navigation, tap the speaker icon at the right of the screen. That should give you the option to mute only the directions (but keep traffic alerts on), or to mute all nav voice alerts.
I have a similar problem since I got a S6 Edge+. The strange thing is though that once I leave the car I start getting directions. I can hear the voice test OK