It's probably a lot simpler than you think - try looking in your soundcard settings to see if the MIDI playback is just switched off, or the volume dropped to nothing.

It happens quite randomly on my XP home and XP Pro machines ... so you may find it isn't a 'permanent' fix, but something you have to do almost every time you want to use it.

Either right click on the volume control icon (bottom right) if it's there, or access it through control panel.