I don't know how they are doing it, but listening to those speakers seems unreal. I opened the Macbook to look at them, the speakers look pretty big and they seem to have a long resonance channel, and I think they are glued to the aluminium chassis which appears sturdier than any previous Mac I've opened.
They are optimized to sound best when you are sitting in front of your laptop, and the volume is of course limited, but there is nothing "cardboard box" like about the new Macbook Pro's speakers.