On my unit (Bog Standard Business CD) I always use this procedure:-
With ignition off.
Hold the 'M' button down and keep pressed while switching the unit on.
Keep 'M' button pressed for about 10 secs.
Cycle through options with '<' and '>' buttons.
Then turn it back on. Within 8 seconds press the "SEL" button and hold for 8 seconds.
If you have the Nav screen the secret menu shows up there. If not I'm guessing it shows up on the radio screen.
The "<" and ">" buttons scroll through the menu selections.
As said before the "Gal" menu is where you adjust the speed volume increase.
This is moved up or down by pressing the "1" through "6" buttons
(1) being the least amount of increase and (6) being the most amount.
Once your through adjusting turn the radio off and back on again.