Truman:

I was similarly stumped with the volume on my 04 XJ8. I discovered that the volume control on the steering wheel controls both the sound system and the Nav. But you can only control the Nav when the voice is actually speaking (or trying to). If the volume is turned all the way down for the Nav, you might never figure out how to activate it. The easiest way to do it is to touch the small speaker icon in the lower right corner of the Nav screen. This gives you audio directions "on command". While this audio is playing (or trying to), simply adjust the volume on the steering wheel. If you try to adjust the volume at any other time, you are simply turning your sound system up or down. As I said, this is how it works on the XJ. It might be different on the S type. My wife has a 2000 S but no Nav system.

 

YEP same on the S Types