Sorry to be grave-digging, but if I'm out here scoping things out, it's likely this gets more views than just mine. My educated guess is that Google Navigation is using the microphone, and it has to do that exclusively when it's in the foreground (displayed). You can't use the camera with more than one app at a time, either. Android, as far as I can tell, doesn't have a feature like AutoVoice that forwards actual commands to apps as intents. It lets apps use the hardware. So, even Google Assistant is set up to commandeer all the voice input, interprets it, and then sends intents to the app (calendar, timer, search, w.e) to procure the action. Google Apps are notoriously greedy and isolationist, given the interference of other apps has so often been what caused things in GApps not to work. (that's why GMaps doesn't use the voices we install as default.) So, unless you want to use a navigation app that will let you override its voice input altogether, or just doesn't have voice operation at all, my guess is you're out of luck. Hit the home button to minimize Google Navigation to the notification area, and it should work.