This is one of those blog posts where I am going to complain about how something doesn't exist but at the same type hoping that somebody will step up and say, no you are wrong, this things has been in the shops in Korea for 5 years and it is not even that expensive. What I want is simple: the same music playing in different rooms.
When we lived in Amsterdam we had this more or less; I had a splitter on my stereo that split the signal four ways and thus controlled for sets of speakers. Each had a button and I had wires running from the stereo to four different rooms. But this kind of wiring is unpractical; I drilled holes in multiple walls and still it was a lot of wire sticking out everywhere. It worked though. Here in Switzerland I decided to go the wireless way. I started out with wireless speakers.
The first pair worked for a while pretty well, then started to degrade and then no longer worked. I bought a new pair and the same thing happened. Maybe I should have spend more money, maybe the thing interferes with the Wifi networking going on the same rooms, I don't know, but I wasn't happy. Today I bought a Terratec Digital Audio Receiver, a little box that lets you stream music from your computer to any room within Wifi range. It is nifty, but it requires a server running on your main PC and you can't as far as I know play the same music through the speakers of your computer as through the Terratec. Partly this has to do with latency (it takes a little time to encode/decode the music that is streamed), but this seems fixable.
It struck me that a much simpler solution would be to use the electricity network. While shopping for the DAR I saw that they have network cards that reach 85Mb/s over the electricity network, so the capacity is there. You would have a sender module that has a digital sound in and a 230V connector. It would take the sound in and put it on the wire. You could then have as many speakers as you wanted anywhere in the house that just have a 230V connector. Plug'm in, select left/right (or of course from a wider x.1 scheme) and you're up and running.