I'm not sure what you mean by "cut off"; I have noticed that when playing audiobooks or podcasts in the "Music" app (I don't use Audible), the voice does not pause, but reduces volume temporarily. Unfortunately, this not only makes it hard to understand either voice (it's like two conversations in a room), but it makes me miss my podcast audio and have to rewind.
It does however continue normally after Waze is done with the direction voice.
Waze 3.1.1.0, iPhone 4s