One problem I see with the idea of using the “radio streaming” approach. Remember that the roku’s use a buffer so they are effectively playing the audio just after it was actually on the network. To me this would mean that unless you started both Roku’s simultaneously they will not be playing the exact sounds at the same time and, if you can hear both at the same time you are going to get a few second echo affect that will drive you crazy.
Yup, and even then, latency and stuff will cause problems on wired versus wireless, etc. It’s a non-trivial problem, and would require the clients communicating with each other to establish a shared clock or something. That’s why it would take code on the soundbridge as well as on the server.