Apple Users

This is a real pain in the ass...

You have to adjust the volume WHEN SHE IS TALKING.

If you are using Bluetooth through the car's audio system, try to get her to say something that will take a while. Set a destination, leave your driveway and turn the wrong direction. When she starts to say "make a U turn", turn the volume up on your sound system before she quits talking. You can adjust the radio volume and the bluetooth volume independently, and that can be a useful feature.

If she's not talking, and you are turning the volume up, all you are doing is increasing the "ringer" volume and not the app volume. Obviously, do not adjust the volume on the phone while the car is in motion.

There is probably some other way to do it, but I know of what you speak.

The other thing I do is look for people at bus stops. Offer them a ride in exchange for having them watch the screen and tell you when turns are coming up.