There should be a checkbox on the pop up saying Don't ask again. If you're not getting the pop up message anymore you can go to manage applications and clear the data and cache for Google navigator. It will give you a fresh install the next time you open the app.
the announcement voice in ice cream sandwich version is horrible. especially in the google maps navigation. It is just Horrible. it resembles a trembling robotic voice. Unbearable to hear when using navigation. I am surprised there is no much comment on this anywhere. I have this in Xoom tablet. But to verify I also checked it in a galaxy nexus with a friend. The voice quality is poor. I am unable to use navigation due to this.
What am I missing here, pl let me know.