Problems converting .mid to .wav, please help!

I recently completed a musical score, in a program called 'Finale 2006', but there's a huge difference between the midi sound and the wav/mp3 sound. It's as if the app uses a whole different midi instrument library to export files to wav, compared to the one for playback. The wav/mp3 one sounds crap, but since I can't upload .mid's to the podspace, any ideas as to what I can do to fix this problem?