Hi K3AN,
Welcome to the Apple Support Communities!
We understand that when you're listening to your audiobook in your car, the alert sounds are the same volume as your audiobook.
In this case, the volume of the notifications and the apps aren't adjustable independently. If you'd rather not hear the notifications while listening to your audiobook in the car, you can enable Silent mode on your iPhone. This will enable to you to listen to your audiobook without the interruption of your notifications. You can learn more here: How to put your iPhone on vibrate, ring, or silent mode
Feel free to share any feedback about this feature with Apple directly here: Feedback - iPhone
Take care!