Try taskbar repair tool, you can download it from here:
http://www.kellys-korner-xp.com/taskbarplus!.htm
** Try "Hide or Show All Listed Icons" under Notification area.
On the missing volume control icon issues, access the sound and volume
devices from control panel and check whether the "Place volume icon on the
taskbar" option is checked.
If that has been tried:
Place your XP CD in then go to Start/Run and type in:
EXPAND -R X:\I386\powrprof.dl_ C:\Windows\System32
Once done: Go to Start/Run and type in: regsvr32 stobject.dll