choose Sort from the menu choices under Text
I assume you meant the "Sort lines" command from the Text bundle =)
I tried to run the Sort command through the Filter through command and got the same result. Empty document.
just to make sure sort didn't rot: open a terminal, then type 'which sort'. it should reply /usr/bin/sort (or even /sw/bin/sort if you have fink installed). now do 'sort', type in some random lines, then press ctrl+D on an empty line (that is, EOF). check for consistent output (it should print your random lines, er, sorted). if it doesn't work, something got messed up.
ciao,
D.