I could solve a sound issue on a friend’s smartphone, by uninstalling Text-To-Speech languages, then download again. This was an issue of very low speech volume mixed with no speech, but just a short click-sound instead of speech. Not very likely in your case, but a 5 minute job.
Edit
Found an older thread:
Today I wanted to test how my second mobile phone (which I normally never use for navigation) behaves when the screen is off.
However, I didn’t have any voice output on this phone at all. When I started it up, I heard the “ping”, but not “Let’s go”.
Here’s what worked for me:
I uninstalled Google Speech and Voice Recognition (the update from late June) and reinstalled the updates. I deleted the voice files and downloaded them again. And it worked straight away.