Welcome to Android Central! First, make sure you haven't muted Google Maps -- there's a prominent Mute button on the nav screen.
Second, is this only when the phone is in your car? If so, is it connected via Bluetooth to the car stereo? Open Google Maps, go to Menu>Settings>Navigation Settings, and make sure Play Voice Over Bluetooth is turned on (and the car stereo is turned on, set to Bluetooth as the source). Or turn off that switch, and make sure the phone's volume is turned up.