The volume icon in the taskbar typically disappears if the system icons settings get changed by accident. This also occurs when the audio device is not properly configured or when the driver software gets corrupted. A user can correct the problem by manually changing the settings or updating the drivers.
Windows XP and newer versions of the Windows OS include the option to hide specific icons from the taskbar. In that case, the easiest way to fix the problem is to access the Notification Area Icons settings via the Properties dialog box for the taskbar and reverse the changes made to the volume icon. If that solution fails, updating or reinstalling the audio driver should set the icon to be displayed by default.