It may, for some reason beyond my comprehension, be associated with an event in your 'Sounds and Audio Device' Control Panel. Have a look in there on the 'Sounds' tab and see what files are associated with the individual events (I am assuming here that you are running XP, otherwise it may just be the 'Sounds' Control Panel). Certain programs will add their own sounds for events so something you have installed recently may have done it.