The only settings in there is the announcement volume and to turn off guidance during a call.
Found something in the setup\volume menu labelled "audio lowering", you can change this from off, low, medium, strong. Not sure if that is what reduces the audio for nav announcements.
That sounds like the jobby. Theres a separate one for the parking sensors. I'll check what mines set to - think low for nav and strong for beepers IIRC>