Don't know if anyone else has mentioned this, if so, apologies.
It seems the car controls volume in at least 2 discrete settings. Specifically, music/podcast/audiobook volume (entertainment) has one volume level and the car itself has a separate volume setting. I wasn't able to hear Siri or the text message read-out or incoming calls even though my music was perfectly audible. I tried cranking up the volume as the car - not the music - was audible and it worked, music level didn't change but the navigation/siri/text message volume increased. So maybe try to get the carplay app to "read" you a text message (it'll be really quiet) and turn up the volume as it's "reading" - dumb, but seemed to work for me and now I can hear texts and nav and phone calls just fine.