Hey there joycefromavon lake,
Thanks for reaching out to Apple Support Communities. We'll be happy to help with CarPlay. While the behavior you see can vary from one manufacturer to the next, causing some differences in the way CarPlay works, you should be able to adjust that voice volume.
If you adjust the volume using the buttons on your iPhone does that make a difference? If not, adjust the volume while the navigation is speaking, and that should change just that audio, without affecting the radio.
The following resource can help troubleshoot CarPlay: Get turn-by-turn directions with CarPlay - Apple Support
Take care.