Hmm... I am not sure there is a commercially available product for this.
This is not something you can just throw together either.
Well you could, but if
you could then you would not be here asking.
Your best bet if you are set on DIY'ing this is to pick up a used set of wireless rear channels speakers. Use the transmitter as is and cobble the receiver/amps out and build them into your speakers.
But in general wireless audio (and video) systems are prone to interference from line noise, RF noise, and the many device on the already over crowded 900Mhz, 2.4 and 5.0 Ghz unlicensed low power radio bands.