There's a bug in Noteworthy whereby if you import a multi-channel MIDI file into MWC, it sets the MIDI volume control to 0.

In that event, until you go into Volume Control and turn your MIDI input back up, the results are exactly as you describe. Volume Control is accessed as detailed by Bernard above, although it's also under Start >> All Programs >> Accessories >> Entertainment >> Volume Control, or by typing
sndvol32
into the Run box.

Apparently Big Brother (Microsoft) is out to destroy the world of MIDI. How can we fight back?

That's one heck of a logical leap you've just made there :)