Hi Mike,
There are definitely a few things that you can try to get this resolved.
The first (and simplest) would be to ensure that all your settings are correct.
Firstly check to make sure the volume is turned up on the iPhone, and test with music or video to ensure its working as expected.
The next would be to open up the Settings App and go down to Maps and select the option 'Driving' for Preferred Transport Type. Next head into the sub menu 'Driving and Navigation' > Navigation Voice Volume, and ensure that 'Loud Volume' or 'Normal Volume' is selected.
If you are still having the issue after that, try some more basic troubleshooting such as force closing the app and reopening it, and turning the device completely off and back on again.
Let me know how you go, and we can look at more options if you don't have much luck with those!