I was able to fix this by resetting the unit. - Press I then reset. If that doesn't work, remove your negative battery cable for a few minutes.
I was also able to get the unit to stop cutting out audio when pressing soft keys with navigation active by muting the "beep" in navigation sound...
I recently installed this in my sons car and initially the voice prompts were working. Then after a couple of days it stopped. I was able to get the navigation voice prompts working again by resetting the system. Just press the I on main screen then reset.
Worked for me!