I don't know about Windows 8.1 specifically but this is a common occurence with laptops or "multimedia keyboards". Do you have a "Fn" key on your keyboard that you can press as the same time as the "Ctrl+F11" or "Ctrl+F12" commands? Maybe your computer interpret the F keys as the multimedia commands by default so you need to use the Fn key to make these keys act as normal F keys again. There is usually a setting in the keyboard manager to revert F keys to their normal function.

Ah, thanks. It was some craptacular Dell "feature" that changed the Fn keys from their normal operation to controlling the hardware instead! So in order to press F1, you had to press Fn-F1 instead! Ridiculous! An hour of dorking around with Google searches and Dell utilities, and I finally had to reboot to disable the "feature" in BIOS. Now I press Fn-Fkey to activate the hardware functions. How it should have been in the first place! Grrr.

Well, I use a HP laptop with Windows 8.1. I don't have a problem with it. As you said, it is likely Dell's proprietary key setup.