In some cars the radio is shut off when your battery level is low, kind of like a power saver mode. I’ve seen cases where the CarPlay display still works but audio doesn’t.
You could try leaving it on a battery charger overnight to see if that helps.
This happened to me a couple times with apple car play and pandora. Car showed it was playing but no sound. I launched the app on the phone, cleared it, then launched it back up and it worked just fine. I don’t think it’s really a issue. Just a sync feature small glitch with Bluetooth/phone apps. Like IT always says, reboot and problem fixed.