F1 for Help, F11 for full screen and so on, those are the normal functions of F keys. On laptops you can use the manufacturer specific alternative functions by pressing and holding down the FN key then pressing preferred F key. In you case I bet that if you press FN + F1 instead of just F1, it works in the way Dell has customized it, as mute / unmute.
Most laptops allow user to set default function key action in BIOS / UEFI settings. In your case it sounds that your laptop was earlier set to give Dell specific functions without FN key and normal functions with it, and now it has been changed to give normal standard functions without FN key and Dell specific functions with it.
(Position of FN key on actual keyboard may vary.)