I'd like to be one of the first to welcome back the waterbed to the world. Akva Waterbeds' Sound bed is a waterbed with an integrated sound system. We've seen beds hooked up for sound before, but what makes the Akva so much better is that the sound system is below the pocket of water. This way the music creates soothing waves that can lull you to sleep. It might be a good idea to avoid playing any kind of rock or metal for fear of a waterbed tsunami.
There's no word on availability or pricing, but if you really want one of these right now I'm sure you could find a waterbed at a garage sale and just slap a couple small speakers underneath the water pocket.