BEFORE YOU TRY THIS!!! …
I had this problem, too, so I made the change. Here's what happened:
First, what was suggested actually worked. When I'm playing an audiobook and Maps needs to provide directions, the volume is now reasonable.
HOWEVER, now Maps navigation doesn't limit itself to Bluetooth!
There are two sound paths when using iPhone in a car: one is Bluetooth, and then other is what you hear when you use Siri. When I didn't want to hear navigation, I would simply turn off my radio. Problem solved. To be fair, I rarely listen to music or audiobooks while driving.
NOW, however, when no other audio is playing, Maps audio plays through the sound path that Siri takes … even if the Bluetooth radio is on! Unfortunately, there's a delay while iOS checks for audio, so I constantly lose the first part of the navigation audio. Instead of "In 2 miles, take the exit…" I get "Take the exit…" That's a very different instruction and has caused me several mistakes.
WORSE is that I've reverted the change listed in this thread, and THE NEW SIDE EFFECTS STILL EXIST!!!
To anyone who's considering this, please use caution. Back up your phone before making the switch so that you can restore if you want to (I'm still not sure it will help).
If anyone has suggestions on how to undo this setting other than the obvious "click the button to off", please let me know.