So when you go back into the Volume Settings, are you saying that the Guidance Volume slider (Low----High) is in a different position than you set it before? I've not had that happen.
Here's the only thing that I've noticed. This may not be what you're asking...
I believe that the radio only turns down a certain amount when the Guidance Voice speaks. In other words, on a Radio Volume Scale of 1 to 10, the radio might turn down by an amount of 2 when the Guidance Voice speaks, regardless of where the radio volume is set.
So if the radio is set at 5 and the Guidance Voice speaks, the radio turns down to 3.
If the radio is at 9 when the Guidance Voice speaks, the radio turns down to 7 (meaning at this volume it drowns out the Voice).
The Guidance Voice is a set volume and does not speak louder relative to a louder radio volume, and the radio does not turn all the way down to a set volume so the Guidance Voice can always be heard...the radio volume only decreases by a set amount when the Voice speaks.
The only other thing I see in the manual (Pg. 5-51) is a way to set the voice guidance for the phone. I'm not certain that this is a subset of (or overrides) the generic Guidance Volume.
If this does not help, I can go out tomorrow and play with it in the car to see if I can help you figure this out.