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My volume icon disappeared after I updated to OS X Yosemite 10.10. I still have sound. resetting the PRAM didn't work.
I have refurbished 2012 Macbook Air
Go to System Preferences then Sound, then put a check on "Show volume in menu bar".
If you're using Bartender, check that it's not set to completely hide the volume icon.
You would think if you had this answer's problem you wouldn't need this answer, but then you wouldn't be thinking of me -_-
Open System Preferences, click on Sound (in the second section, all the way to the right), then make sure Show volume in menu bar is checked.