You can restore the volume icon on your screen through the control panel or via the system registry. The process differs depending on the operating system.
In Windows XP, click on the Start button, select Control Panel then the Sounds and Audio Devices icon to open the Volume tab. Check whether the "Place volume icon in the taskbar" option is checked. Select uncheck and click Apply. Select check, and click Apply again to display the icon on the taskbar.
In Windows 7, 8 and Vista, check to see whether the volume icon is in the grouped icons and drag it to the taskbar. If it is not, right-click anywhere on the taskbar and select Properties. Click on the Customize button under the Notification area. In the open dialog box, change the Volume icon's behavior to "Show icon and notifications". At the bottom of the screen, select "Turn system icons on or off," and ensure the Volume icon is set to "On". Click on Apply and the icon will appear on the taskbar.
You can restore the volume icon together with other system tray icons through the system registry. Click on the Start button, and type "regedit" in the search bar. In the window that appears, go to the Edit tab, and click on Search. Type IconStreams and click Next. Delete the resulting files, and restart your computer to display system tray icons.
If the volume icon keeps disappearing, update drivers for the soundcard through your computer manufacturer's website.