The Silent Treatment

The navigation system in my 2010 Prius usually gives voice prompts, e.g. "Next Right", "Continue on I-880 South" etc. I can even elicit one by pressing the "MAP" button.

But every once in a while, it gives me the silent treatment. The turns are all shown on the screen, but the voice remains silent. Pressing the "MAP" button does nothing. When it gets into this mode, it remains silent. I can fix it as follows: I delete the destination, park the car and turn it off. Then I re-start the car and re-enter the destination again. After that, it speaks normally.

This problem has only happened about four or five times in the year and a half I've had the car.
When it happens seems to have to do with when I set the destination.... I've experienced it when I hit "go home" after I'm already rolling down the road toward home. But not every time!

Has anyone else experienced this problem? Can you shed any more light on it?

- Rich