MacOS – My volume icon disappeared after I updated to Yosemite.

This can be caused by the default sound settings being changed. If you go to System Preferences --> Sound --> "Sound Effects" you will see a pull down list labeled Play sound effects through... On my machine there was the following listed: Internal Speakers and my monitor.

For some reason after the OS upgrade my monitor (and HDMI) was selected instead of internal speakers. Switching to Internal Speakers fixed this. You should select the appropriate item for your setup. Also, you can go to the Output and Input tabs and verify that the correct item is selected as well. See this picture for additional information:

Got around to looking at this again, with some searching on the WWW and the assistance by telephone with a 'senior' apple advisor.

What I found on the WWW was reports of com.apple.geod showing "not responding" in Activity Monitor. This process is needed for Location Services, so if it is crashed location services won't work.

Force Quitting the process in Activity Monitor seemed to enable Location Services, but on restarting the Mac com.apple.geod crashed again.

So (on the advice of Apple) I removed com.apple.GEO.plist from MacBook Pro HD▸ Users ▸ username ▸ Library ▸ Preferences (from hold down option Finder>Go>Library then select Preferences - if you don't hold down option, Library is a hidden folder). Then I restarted the Mac. This time everything seemed to work. So maybe it is just a corrupt .plist file. I get the "Location" icon in the menubar now.