Concrete speakers have been surfacing since the 1950s. They are a terrible idea, and not worth the time of day. Good old Gilbert Briggs debunked that in the 1950s!
Yes, they are rigid and don't vibrate, but they have one huge problem, concrete is a selective band pass filter. In other words some frequencies pass through the concrete easily and others don't. So it is not new and still a bad idea, in fact a very bad idea.