Ive looked through at least what I think to be all the settings. I cant seem to find a way when using CarPlay to not have alerts such as ringtone and text tone to come in at the same volume or seems louder than the music. When I get a phone call or imessage just about makes me jump outta my skin. Is there not a way to turn them down other than putting the iPhone on silent.
Sadly, there isn't such a setting. With iOS 11, you'll be able to mute notifications when driving, which should help.
There is a seperate alert Volume. Can't remember if it affects the ringer or not or it's just for the sat nav.