Thanks for reporting! Voice instructions are indeed in an early stage and therefore not configurable.
When the app goes to the background (locked screen, other app open, etc.) navigation is paused and therefore voice instructions are killed too. The app must be open (for now obviously).
While actively using the app, the screen should not be locked. This can still happen when the OS deems this necessary (high temperature, low battery etc).
Changes to this will be made!
