SOLVED! - When all else fails - read the instructions!

Who would have thought that a "setting" called "Nav Volume" actually has no impact on the Nav Volume!!!! - but that it just affects the volume of all other audio sources in relation to the Nav Volume when the Nav. System is giving an instruction.

Nav volume is set by adjusting the "Nav min" and "Nav max" settings between -9 and +9 accessed by pressing the "Info" button on the top right of the display, and then adjusting the "Nav-min" option.

Could they have made this less intuitive???

Thanks to those who helped.