I have a blind relative so some awareness of voice activation, but have not heard of a camera. My guess would be the first software for it would be by way of a computer connection rather than in camera. Assuming your people are sighted, you might look into the capabilities of the new touch screen models, that are offering various amounts of functionality by simply tapping or stroking the LCD - it will be getting better over time no doubt.
By coincidence, this morning I came across an interesting piece about a blind photographer and her working collective in the New York Times:
Here's the link to the group she works with:
I'd imagine these people would be more on top of the available technology. Best wishes for your worthy project.