Tried that no go the a solution I found was when you are using the native GPS there is at the left a volume symbol that pops up sometimes tap and there are sliders to set the volume. There may other ways as well but this one works.
Turning up the volume when it's speaking PLUS having the radio on will raise the Nav Guidance volume. It won’t always do it with just the volume control if the radio is off. Doesn’t make sense but that’s what I’ve observed.