Mp3 use

According to the XT6 book I should be able to plug my mp3 into the USB port in the console and listen to it. The mp3 worked great on the Denali that I traded in. I have taken it to two dealerships and talked to a corporate tech. They have told me that I might have the wrong kind of mp3,but can’t tell me the right kind, then suggested downloading mp3 to a flash drive that didnt work either. They then suggested other methods of downloading mp3 to other equipment none of which worked. Any ideas? It’s really irritating to not be able to use the mp3 especially when it works on a kia.