Volume shortcut for Magic Keyboard not functioning

Hi,

I have a Magic Keyboard. F10, F11, and F12 have symbols to adjust the sound on my iMac, but they don't work. Instead F11 shows or hides the desktop. F12 shows or hides the dashboard. I want to control the volume using F10, 11, & 12 from the keyboard. How do I get these keys to control the sound volume?

I have an iMac with High Sierra 10.13.6

Thank you.