If you have missing volume icon in system tray on Windows 10, then you should run Registry Editor, with open Run by typing Win+R buttons. Enter "regedit" which means we are editing registry. In here select key
HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\Current Version\TrayNotify
Here find couple of registries name IconStreams and PastIconStreams.
Select those and delete them if exists.
Restart Windows Explorer or MS Windows, and your volume icon should be recovered.