Play/Pause/Prev/Fwd buttons not working

What I have: OS X 10.10.2 on 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.

Where should I look to fix this? Notice, the issue happened before updating to OS X Yosemite.