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Check the on-screen volume control. When you've already started a route in Apple Maps, tap the screen and you should see a volume control in the upper right corner.
The only other thing I can think to check is to set the volume with the car's volume controls. This is separate from the radio volume, or whatever music you have playing. When you know Apple Maps should be speaking to you, increase the volume with the car's volume **** or steering wheel button. You have to catch it just right. If your background music volume increases, then you didn't catch CarPlay when it was speaking to you. It's a bit frustrating, but that's the way Apple Maps seems to work with CarPlay.
An easy way to adjust that is to ask a Siri a question like what time is it and then while she responds, turn the volume up