I'll take the risk of embarrassing myself here, but have you tried Fn + Esc
While this made no difference to my Ubuntu 14.04 on my old laptop, on my new Ubuntu 18.04 installation,
the Fn + Esc switches the functionality of my Brightness and Volume Control buttons on and of.
Yes, I noticed that. Very odd, I have to say.
I found out that my arrows (up and down) control the volume, whereas the brightness is controlled my F11-F12, as per Dell's design, but with a huge delay (from a few seconds to several minutes).
The least I can say is that it is all very weird.