It is common that system tray icons disappear even though the apps are running. But in the case of volume control, it is crucial for me to have the icon always there.
...Right click the task bar and then click task manager and then click the processes tab then fine 'EXPLORER.EXE' then end process then you task bar should disappear after that click on the applications tab and then 'new task...' and type 'EXPLORER.EXE' it should appear ...