Here’s the solution, courtesy of howtogeek.com
It’s a very simple registry edit but if you’re not confident editing the registry then don’t attempt it.
1. Back up the Registry by creating a restore point.
2. Go to Start > Run (or Windows-key + R), type in regedit and hit OK.
3. Navigate to the key HKEY_CURRENT_USER\Software\Cla sses\Local Settings\Software\Microsoft\Wi ndows\CurrentVersion \TrayNotify.
4. Delete the values IconStreams and PastIconsStream.
5. Open up the Task Manager (Ctrl + Shift + Esc), go to the Processes tab, select explorer.exe and click End Process.
6. Open the Applications tab and click New Task at the bottom-right of the window.
7. In the message box that pops up type in explorer.exe and hit OK.
8. Explorer.exe will reload, and the missing icons should now be back in the system-tray where they belong.
9. Then if the volume bar isnt there, go to taskbar properties (where the volume was gray) and simply tick the box.