Maybe the settings in Goolge maps is set to mute? On the top search bar, there are 3 parallel lines. Click on that and it will bring up a settings list. At the top right there is a gear icon. Click on that and then click on navigation. Make sure unmuted is checked. FWIW, I have my guidance volume set to louder and play voice over bluetooth is activated.iPhone X, upgraded to iOS12. CarPlay works after the update and Google Maps works too but somehow i do not have sound for turn by turn navigation on Google Maps.
Of course you should also turn your radio volume up