Since SmartMusic doesn't use MIDI directly, I'm suspecting that you may have posted this question to the wrong forum. If you're using Finale, Allegro, Guitar or PrintMusic, please repost your question to the appropriate Notation Forum (appropriate meaning the Mac Forum if you're using a Mac or Windows Forum if you're using a Windows machine).
You may also want to include more information about your setup: what version of the software, what version of the Operating System, what are you using for playback (i.e. soundcard, MIDI Keyboard or Sound Module), and what are your choices in the MIDI Setup box.