How can I toggle the Fn (function) key?

To toggle F1 .. F12 keys use media functions or to be the "F" keys press Fn + Esc .

If you have an Apple keyboard and want the function keys to act like F1-F12 by default (disable Fn default behavior) give the following command in terminal:

echo 2 | sudo tee /sys/module/hid_apple/parameters/fnmode

from here: https://help.ubuntu.com/community/AppleKeyboard#Change_Function_Key_behavior

As far as I know, the Fn key does not get detected by the OS but modifies your other keys on a firmware level. That means you can neither retrieve nor control the state of the Fn key from Ubuntu.

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What you could do instead is to change the keyboard shortcut for increasing/decreasing audio volume to an alternative key combination instead of the Fn-modified special key.

Open System Settings → Keyboard and go to the Shortcuts tab. In the list on the left side, select Sound and Media.

After clicking on an entry in the list on the right side (in your case you want to edit Volume up and Volume down), you can press a combination of keys you want to assign to this shortcut. Esc cancels the editing and ← Backspace disables the shortcut.