Here's what worked for me on my Samsung Core phone with Android:
1. Hit your volume 'up' button and then check the 'Media' is turned up. (I know... just check it anyway.)
2. Uninstall Google Maps app. This deletes all caches, etc. Re-install Google Maps. Mine actually only 'Updated' when I did that, as I think Google Maps is a default app on newer Samsung phones.
3. Ensure phone 'data' is turned on. 'Maps' will not work beyond the range of WiFi without data turned on.
4. Go to your settings and turn off Bluetooth. (Sorry, your phone won't link to your car now, but that's what you give up to have voice come out of the phone itself. Not sure if voice will come out of a Bluetoothed linked car's audio system, but you can try that later.)
5. Start Google Maps app and put in a route. (must have Origin and Destination.)
6. Click 'Start Navigation', or the blue button or arrow, or whatever. This should bring up the 'Green' header at the top. If the voice is working, you may hear something at this point.
7. Tap the 3-dot vertical bar at the lower right and select 'Settings'.
8. Uncheck the 'Play voice over Bluetooth' box. This forces the audio to come out of the phone.
Now you need to do something that will cause a voice command - drive around a corner. The voice should now come out of the phone.
Good luck!