I have no idea, the voice doesn't work on my Nav4All either. There is something on the Nav4All knowledge base you can follow to make sure the voice file is downloaded, but having followed it (more than once) it made no difference.
The
KB article (KB110025) states:
Check the following:
1. Are the language files downloaded correctly?
Go to "Settings > Choose language > Display Language" and select your language.
2. Make sure the volume of your phone is high enough.
3. Make sure your phone is not switched to a "silent" profile.
I raised a ticket request, but not heard anything yet. I'm sure I've read elsewhere though that you're unlikely to hear anything, but thought it was worth trying, just in case.