Similar issue during five hour drive today. New 2020 Silverado 2500HD custom, had CarPlay going running Waze and couldn't hear the voice prompts. Turned the volume up and got the prompts but also go the FM radio as well. Tried everything I can think of to turn off the radio and just have CarPlay running. I've only had the truck two days so I'm probably missing something stupid. I never listen to the radio, so five hours of FM and static was not fun.