Bear in mind two things:-
1) The sound volumes for the audio system and nav system have separate settings. If the nav volume is low, a speed camera warning (which is quite brief) may not be heard, but the main audio sound will dip anyway, so it may sound a bit weird briefly.
2) There's a setting in the audio setup called audiopilot. The default is off, I believe. It's a speed/background noise dependant volume control - if on, it changes volume automatically depending on what speed you are doing, etc.
Alan