In case you want to use only F11 and F12 key without the combination with FN key to increase or decrease the volume, the setting has to be changed in the BIOS screen.
Restart the system and at the Dell logo tap the F2 key every two seconds. In the setup screen click on ‘Advanced’ and highlight ‘Function key Behavior’ and from the pop-up box change from Function key to Multimedia key. Press F10 to save and exit.
Please revert for further queries.
Thanks and Regards, DELL-Sujatha K Dell Social Media and Community Professional #iwork4Dell