That used to happen to me from time to time with Melody Assitant running with windows XP. Not running that now so I can't give the exact menu titles, but try going to control panel/sounds/ - it might be the "advanced" tag. On mine this bought up a panel of five volume controls including one for, as John suggested, midi sounds. Check that hasn't been muted. Apparently there is a bug where running one app, possibly real player (?), causes the midi to mute.