IOS 15.3.1 Car Stereo via USB.

Check your car'd radio "Source" settings for "iPhone" or more likely "ipod" and the chances are only music will play on it.

If your car has Apple Carplay then you need to add approved apps in Settings, General, Carplay

I too had this issue. My previous iPhones connected through USB / AUX cable without issue. New iPhone 13 Pro Max will not. My car had an outlet for a speaker line out (just discovered this). Purchased a Flat Auxiliary Audio Cable for $20 and an iPhone Dongle the Apple Lightning to 3.5 mm headphone adapter for another $8 and my problem has been solved. I hope this is an option for you!

Had the same issue with USB after update in my 2010 Camaro, I read in another chat about CarPlay, which wasn’t the USB problem, and Bluetooth and it said to turn off “contacts” in Bluetooth settings. I thought it didn’t hurt to try and it worked! My Apple Music began as soon as I plugged in the USB. I hope this helps others, it is frustrating