Had the same problem once it started to get hot here in Chicago. System worked fine while it was cool, but after sitting in the hot sun, NO SOUND and no control. Couldn't hear anything, no source selection, no control of the power button. System seemed to be working because the nav. displayed the correct location and you could watch the song titles change, but no sound.
Took it to the selling dealer and showed them phone video of the problem and they had it for 4 days. Replaced the APIM module. Dealer said this fixed the problem. Drove the car home and system worked. Next day after sitting in the hot sun, same problem. Drove car to my local dealer and showed them the problem.
Took car to local dealer, they had the car for 4 days and were finally able to replicate the problem. They replaced the audio control module (ACM) and returned the car to the parking lot and left it sitting in the hot sun. When I picked up the car last night the system worked fine. Re-paired my phone and everything worked.
We'll see how it works now that I have it back and can fully test it in the real world