Volume/mute/brightness keys stopped working

[Ian Pilcher]

I have a Dell Precision 5520 laptop (basically an XPS 15).  The volume,
mute, and brightness "function" keys have always just worked on this
laptop with no additional configuration required ... until now.

Something has changed in the last week or two, and the keys no longer
have any effect (except that the brightness increase key now toggles
the maximized state of terminal windows).

Anyone else seen this recently?

Assuming no, anyone have any hints on how to debug this?  I'm not really
sure how these function keys work (i.e. are they keypresses, ACPI
events, something else?).

Any thoughts appreciated.  Thanks!