Customers ask me quite often if there are wireless speakers. The answer is yes but with a caveat. Home theater in a box set ups sometimes use line of sight wireless rear speakers. This is entry level stuff and something I don't recommend. Aside from the inadequate sound reproduction the idea that if the line of sight is upset then the sound goes away. Of course, between the receiver emitting the signal and the speakers is you and your couch.
There's also the 900 megahertz technology used in cordless phones. There are speakers that use the same RF signal (not line of sight) to get sound to a remote speaker. 900 megahertz makes for lousy sound.
There's also Sonos which isn't exactly wireless speakers but rather a wireless source. And, anyway, it's not used for home theater surround.
I've written about Summit and there invention of wireless speaker technology for an av receiver for a few years now. Their invention won best new product of the year in Popular Science a couple of years ago. Then last year Aperion incorporated the technology into an av receiver. Now there is such a thing as wireless speakers. These speakers are amplified and thus need an AC outlet near each speaker though.
Here's a short video on how it works.