Relevant hardware: new Mac Pro, aluminum wired full keyboard, external speakers (Line Out). Every now and then, the volume up & down keys (F11 & F12) stop working. (Usually seems to happen upon waking from sleep, but not certain of pattern yet.) The volume keys still make the translucent volume graphic pop up, and they still make that pop-up's slider move up and down, and they make that feedback/confirmation "blip" sound grow louder or softer. BUT, this has no effect on the actual volume (except if I move it all the way down to Mute; that works). I can only change volume with the mouse -- in the menu bar or in System Preferences. The keyboard volume buttons have no effect on those sliders, when this problem arises. All the other F-keys continue to work properly, so it's not a general keyboard or F-key issue. ONLY volume up/down. If I restart the computer, the problem is fixed. And I just noticed that if I go into System Preferences and change the Output to "Built-In" and then back to "Line Out," that fixed the problem too. At least, for now. But I'd really appreciate any suggestions for how to prevent this problem in the first place -- or at least what to investigate as the cause. Thank you!