Let me try to give you a run down of the options for playing CDs and DVDs...
If you use the CD changer with the Nav DVD in its slot, you can play any of the six CDs in the changer while watching the map and using the navigation functions. The voice recognition system can also be used for simple CD commands as well as the full suite of nav voice activated functions.
If you remove the Nav DVD from its slot and insert a CD, you can use that player to play a seventh CD. If you do that you will not be able to look at maps or use the voice recognition system. That is because the Nav DVD has much of the software needed for the map and voice recognition systems.
You can also put a video DVD in the slot where the map DVD normally lives. In this case you will have DVD video and audio output when parked and have the audio from the DVD when driving. Since the player is reading a video DVD, there is no map or nav features and the voice recognition is disabled.
In all of these scenarios, the following will not work: MP3, DVD-Audio (DVD-Audio layer, but DVD-Video layer will work), SACD and DTS CD. Any Dolby Digital source will work in the top player but I have not tried a Dolby Digital CD in the changer, yet.
Hope that helps.