There are audio settings accessible through the head unit where you can adjust the volume for the various audio inputs (microphone, aux, etc). That said, I cannot figure out for the life of me how to sync the volume of CarPlay with my radio or bluetooth volumes - CarPlay sometimes takes a few minutes to come on after I turn the car on, and I get BLASTED out of the car by the volume when it does turn on all of a sudden!
I have my rings and alerts set to midway on the phone itself. To change the CarPlay alert and voice volume, the best way I’ve found is to make the adjustment when Siri is talking. When you do that, you’re adjusting CarPlay and not the main volume. That’s how it worked on my last CarPlay stereo. I’m fairly certain it’s the same in the Subaru.