1. Open Task Manager (Ctrl + Shift + Esc)
2. Click "Processes" tab, select "explorer.exe" and click "End Process"
3. Log off and log in again ...
http://answers.microsoft.com/en-us/windows/forum/windows_vista-desktop/volume-icon-missing-in-system-tray-windows-7/d00ec86b-79fc-4615-a27d-bf83aed35fca
