I have gone through normal steps to right this ie checked for tick in master vol. control.----right clicked task bar then "customise" where i find no representation of an icon but in past items all the icons have turned into folder icons. clicked volume and tuned it to always show but no luck. Tried sys. restore which puts it back until the next time I boot PC
One of our laptops does the same. It's as if in loading icons for other (always on) applications, XP runs out of puff and doesn't show the volume and other icons. I solve it by unticking "place icon....", closing and re-opening the folder, then re-ticking it. I then re-appears. Take it you don't have "Hide Inactive icons..." set ?