did you try doing them separetly like, click on guitar then "one" of them music notes and then Polygons -> Booleans -> Difference. you cannot really do two notes at once i think. if it still does the same thing, try just click on your guitar then "music note" do not select because you may have accidently copied one of them objects under another which you cannot see and also makes it disappears.

goodluck, marlon