I've gotten caught with this once or twice. And I agree that adjusting the Nav volume is easy using the volume knob IF the navigation is audible. IE, if it's too loud or a bit too quiet. If it gets turned off, however, there's no way to know just when the navigation is playing, so that makes adjusting it via the volume button a guessing game. When that happens it's much easier to go to the navigation settings and adjust it up from there.
An easy way to get there is to press the NAV button, Press the /\ button in the lower left-hand side of the screen, then press the button that looks like a gear, then click guidance settings, then adjust volume.