Oono Transmita offers generic wireless audio link: The paired receiver and transmitter will pass audio from any stereo output jack, such as that used for headphones, to a like input jack--say on a stereo receiver with great speakers. While there are plenty of audio devices on the market that stream digital content from a computer or via Internet radio to a stereo system or through built-in speakers, this is a truly generic audio bridge. It uses 2.4 GHz, but isn't Wi-Fi. The transmitter can be powered via AC or USB (though no data passes); the receiver requires AC.
The paired devices is sold for £34.95 via a UK firm that specializes in digital audio broadcasting (DAB), the terrestrial digital radio standard widely used in the UK and barely used throughout the Continent. (In the US, an incompatible technology known as HD Radio is just making inroads. I've started an HD Radio blog to document those inroads.) The item doesn't appear to be available to ship to the US from this store because of that DAB focus.