I'm afraid I'm at a loss to explain it unless it is some kind of issue with "User Profiles" (any others besides yours?).
Do either of the two "workarounds" mentioned in that link work?
By the way there is more than one kind of "volume icon" I discovered this when first launching "Music Match" one day. Somehow it deleted the volume icon associated with my Sound card (Turtle Beach) and replaced it with the Windows sound volume which doesn't have the same bells and whistles. The TB icon faces right and is blue. I forget exactly what the Windows one looks like, but it was different, which is why I immediately noticed it.
You may need to reinstall your Sound Card software, or at least identify the tray application associated with it.
For example, mine is:
TraySantaCruz = C:\WINDOWS\SYSTEM32\tbctray.exe
This was actually deleted from the registry by Music Match and I had to manually create a new shortcut for it and place it in startups.