The rear speakers in my 2016 w/ the THX system are almost inaudible when you have the fade settings set to "0" or the mid-point on the adjustment screen. Since it was only recently I ever sat in the back when the radio was on, I have no idea if this is how it has always been since I took possession a year ago. You can manually set the fade to where you hear them when listening at a normal volume, so it doesnt appear to be bad speakers.
Further, I brought the car into my local dealer for unrelated small issue that turned out to be a fuse. I asked them to look at the speaker issue as well and the dealer explanation was inconclusive. The new service manager didn't have the info on hand when I was picking up the car (after 5 days) and was clearly a non-technical person. She told me they were going to order a new "radio" (her words, !!!) and call me when it came in. That was over 2 weeks ago, haven't heard a thing. Since it took them 4 days to find a blown fuse and come up with this non-diagnosis, I'm really not looking forward to taking it back for something so minor. Overall, this service department has been a real disappointment.
That said, has anyone else noticed this in their MKZ? Could it just be how the balance/fade is programmed?