Using Android v3.7.2 I have repeatedly lost my navigation prompts while still getting alerts (e.g. "Watch out: Rain Ahead"). The only way I have found to fix this is to remove and re-install Waze.
Android v3.7.2
Motorola Droid Razr w/ Android 4.1.2
Android v3.7.2
Motorola Droid Razr w/ Android 4.1.2
Lost Voice Prompts