HWT

I just got this same vehicle and cannot figure out how to get the rear entertainment audio to play on the vehicle speakers. When I select audio from the front seat screen and then select the media source from the usb or HDMI ports, it will not play the audio. Just silence. Also, if not using the gmc headphones, how does one adjust the volume of the rear media? There doesn't seem to be any option to adjust the volume on the screen. I wish they had left traditional audio inputs on the screens. The bluetooth headphones are such a pain to keep charged for the kids.