To restore the volume control to the taskbar of a computer running Windows XP, click the Start button, select Control Panel, and click Sounds and Audio Devices. In the Volume tab, check the checkbox for Place volume icon in the taskbar, and click Apply. If the checkbox is already checked but the volume control icon doesn't appear in the taskbar, then uncheck the option, click Apply, re-check the option, and click Apply again.
For computers running Windows 7, click the Start button, enter "icons" in the Start Search text box, and click Customize icons on the task bar. Locate the icon for Volume, and select On in the Behaviors drop-down list. If the volume control icon still doesn't appear in the notification area, click the Start button and enter "regedit" in the Start Search text box.
In the Registry Editor, locate the registry subkey entry for HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion\TrayNotify. Click the subkey, locate the IconStreams entry in the Details pane, and select Delete in the Edit menu. In addition, locate and delete the PastIconsStream entry. Once deleted, exit the Registry Editor.
Press the Ctrl, Shift and Esc keys together to enter the Task Manager. Select the Processes tab, click the Explorer.exe process, and click End Process twice to end the process. Click New Task (Run) in the File menu at the top, enter "explorer", click OK to restart the process, and exit the Task Manager. Click Start, enter "Customize icons" in the Start Search text box, and click Customize icons on the task bar. Locate the icon for Volume and select On in the Behaviors drop-down list.