I’ve never seen a wireless system that was designed for specifically multiroom. I’d suggest Googling “wireless speakers” for some options. Just make sure whatever you decide on has sufficient range to reach the rooms you want them to go in, and has a substantial number of positive user reviews. Some of them are prone to noise, interference and drop-outs, not to mention the typical 2.4 gHz operating frequency might nuke your cordless phone or wireless internet.
Of course, you’ll still need a “front end” – AM/FM receiver and CD player. You’ll probably have to get that separately from the speakers. Make sure whatever you get has provisions to accommodate the wireless speakers you choose.
Regards,
Wayne A. Pflughaupt