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There is a setting (as mentioned by the above poster) to mute the audio for navigation. When navigating somewhere, click the gear (settings) icon, and then you should see a speaker icon that has a slash through it (because yours is likely muted).
I keep my navigation in the muted state, because I navigate back and forth to work for traffic, but dont need it to talk to me. I go in and turn on the audio when I am navigating someplace that I dont know.
Had the same issue last week. I would turn the voice directions on, but they would stay off, and the icon would revert back to showing muted. Clearly a software bug.