I had this problem, too. After trying so many of the different (and even somewhat complex) solutions to no avail, I found my solution and it was so simple.
When I would right-click on the taskbar and click 'properties', the volume and system icons were greyed out so I couldn't click on them. To fix this, I simply right-clicked in the taskbar area, and unclicked 'lock taskbar'. Then, I was able to go to 'properties' and the volume and system icons were no longer grey so I could click on them.
I'm not sure why they disappeared in the first place, but hopefully this will also work for some of you.