I think a WiFi network would be the only wireless option. I don't think you would achieve acceptable quality through normal radio waves such as those found on baby alarms.
Apple sells a product that allows you to stream your music from itunes around the house wirelessly click here. Depending on the layout of your house you could use a combination of these and then wired connections for rooms close together (eg in our house we have the airport going to the hi-fi in the dining room which then goes to a splitter that can play through 3 sets of speakers. We have one set in the kitchen and another in the dining room).
A better but very expensive option - click here