I thought it was the lower frequencies only that you would hear through the normal decoder. There are two streams, one is a low freq mp3 stream and the other carries the info that allows reproduction of high frequencies. Higher frequencies are very expensive in terms of bits to encode with the mp3 codec. Thus it makes sense that this is what mp3pro could save space on and make the files sound ok at 64k.

I got my info from here: http://www.digit-life.com/articles/mp3pro/index.html

Sean