I recently had COMAND retrofitted and also had the same problem occur. I find these irritating little things maddening. Whilst the route guidance is usually good, the user interface is nowhere near as intuitive as the system in my old BMW.
I think I stopped these announcements by pressing the mute button. It may have been a quick press or a longer few second hold press since it has two functions and I'm not sure when to press it or in what screen.
And I may be wrong. I've just tried to look it up for you in the handbook but no luck. Hope that either this works or someone better informed provides the answer
Yes, all sorted....put the unit into FM Radio display mode, and press the mute button for 3 secs, the TP goes from Red to Black to indicate it's turned off...