The idea of broadcasting media to players in different rooms of a house isn't new. I first did this decades ago using a low power FM transmitter and old school boomboxes with the audio source eventually being a streaming devices over time. Extending this to networked devices was a logical step when that technology became avaialble. I still use Logitech (Slim Devices) Squeezebox radios, devices which have been capable of costreaming from the outset although this was first conceived as a way of pairing mono devices to get stereo. These were first on sale over a decade ago.
Amazon offers multi-room audio with their Echo speakers (also stereo pairing for their Echo Studio units).