What I have: macOS 10.10.2(Yosemite) on an iMac mid-2011 + original wireless keyboard.
Suddenly, buttons for controlling iTunes (they are located on F7, F8 and F9 buttons) stopped working. Meanwhile, other buttons (volume, brightness, launchpad, etc.) still work.
The keys work in other applications but not with iTunes.
Where should I look to fix this? Notice, the issue happened before updating to OS X Yosemite.