I have come to the conclusion that this feature does not exist on my 2006 base coupe. The volume never changes when I'm driving.

Do the base models have an amp? I do not believe they do.

I spent some time fooling with the radio in my base coupe. I was unable to find any options that would allow for a decible adjustment.

I tried many things in an effort to get a hidden menu option to appear. I tried pushing and holding buttons for a few seconds. I also tried pushing multiple buttons at the same time.