Pretty sure this is normal, my 16' CX-5 Nav system does the same thing when in use. I'll have to double check, but I think when I turned the Volume all the way down on the voice guidance, it stopped cutting the sound out.

 

So, after testing with the Turn by Turn/ guidance volume turned up/on and off/down, It turns out it is the Turn-by Turn that triggers the drivers speakers to turn down. And, what I thought was the on/off button, it does, indeed, mute the stereo. Now, my next post is going to be about entering address in the Navigation system using the touch screen. That is seriously annoying. We can and do use the voice control, but the touch screen does not work they was it's stated in the manual or the Mazda website. I'll post a new topic on that. Thanks everyone! You made a difference.