When I play back my song in Noteworthy Composer, it sounds perfectly ok. But when I use another program (eg. windows media player) to play the song, it sounds really distorted! What is wrong?
Check to see that you have the same device set in your midi mapper. (Midi mapper is just Windows' way of assigning a "default" midi device.) Go to Control Panel --> Multimedia --> Midi tab and select the same device as you have set up in NWC.
... To learn what is selected in NWC; go to Tools > Options > MIDI and see what the topmost device in "Devices used by playback" is.