Sound keys on iMac El Cap v. 10.10 no longer work; sound stuck at medium/strong high -- will not adjust sound. Sliding bar now located in upper-most menu....a hassle to get to, click on the bar to adjust.

Sound keys on iMac El Cap v. 10.10 no longer work; sound stuck at medium/strong high -- will not adjust sound. Sliding bar now located in upper-most menu....a hassle to get to, click on the bar to adjust. Keyboard commands F10, F11 & F12 have normally worked; "Preferences not does help". At same time, my MacBook Pro, about 2 feet away, starting doing the same thing (presumably because I had not disabled the feature that lets one computer automatically change the same command by following keystrokes (correct)? How to restore keystrokes F10, F11, F12 to iMac and Macbook Pro? (Adjusts sound louder, lower or mute? Neither wireless keyboard nor larger USB connected keyboard make a difference; same problems.