Its definitely possible, years ago no one ever thought of using solar energy to power a cellphone, today they've found a way to do so. So to answer your question really anything is possible, as long as you have enough funding (lol) to do the research. But just imagine though, if you can power things using voice, it would be kind of creepy, because someone would take those means to make some sort of bomb or some sort of illegal device to oppress others. What may be good for the people can always be turned into some sort of means/device for oppressing others... really blows when you think of both positive and negative effects.

Actually its funny, this is the 3rd time this week I've heard of this, a dude i know at the local university is writing up a thesis on what he thinks using sound energy to power a battery. Its quite interesting hearing him talk about it.