There are some projects like Snapcast[1] or SoundSync[2] (disclaimer: I'm the creator of Soundsync) to let multiple devices communicate together on the same network. The transmission-side isn't that complex: you choose an audio codec, transmit chunks of data and add a synchronization layer (to keep multiple outputs in sync and to correctly delay video playback to match the soundtrack). The bigger problem is building an ecosystem big enough to make it attractive. Bluetooth sucks but is everywhere.

[1] https://github.com/badaix/snapcast [2] https://github.com/geekuillaume/soundsync