Ok, kind of unintuitive, but we JUST had this problem with a brand new Chevy Equinox with CarPlay.

We plug our phone into the USB slot and get Maps going on the head unit. We hear Siri VERY QUIETLY giving the directions.

You have to turn the volume knob on the radio head unit WHILE THE VOICE IS SPEAKING and we can see that the display says 'Voice Volume.' As soon as the voice is done speaking, if you keep turning the knob, the display switches to 'Audio Volume.'

tl;dr There are two separate volumes for the head unit and you can only change the voice volume while the voice is actually speaking. Turn the knob (or press the volume up button) on your head unit while the voice is actually speaking (or during a time when it should be speaking if the voice volume is set all the way to 0).