I am also unable to hear voice navigation over bluetooth while listening to the Audible app, and only in my truck (and my last truck, and the truck i looked at buying, and my wife's car).
Interestingly enough, i do not have the option to Play voice over bluetooth, nor do i have the option to silence voice commands. These options were removed over time through various updates to Maps. The only solution i have found is to carry an external bluetooth speaker and connect to it instead of in-dash stereo.
I wish google would fix this. But i am also smart enough to know that google doesn't care / won't bother.