The volume icon usually disappears from the toolbar when a user accidentally removes it from the customization screen for the notification area or when the Explorer.exe process gets corrupted while Windows 7 or a newer version is running. The icon can also disappear if certain registry values, including IconStreams and PastIconStreams, get corrupted or when the SndVol32.exe file is missing from the system folder.
The problems that cause the volume icon to disappear commonly involve restarting the computer or logging off. The most common problem involves altered settings in the customization screen for the notification area in Windows 7 or Windows 8. To fix this problem, right-click anywhere on the taskbar, select the Properties option from the context menu, and click on the Customize button in the Notification Area section. Find the volume icon, open the drop-down menu next to it, select the Show Icon and Notifications option, and click OK. Click on the Turn System Icons On or Off link, select the On option next to the volume icon, and click OK.
To fix the problem by restarting the Explorer.exe process, right-click on the taskbar, click on Start Task Manager, right-click on Explorer.exe, and select the End Process option. If the process doesn't restart automatically, start it manually by clicking File, selecting New Task, typing “explorer.exe” in the appropriate field, and pressing Enter.