If I press one of the two, I would expect the volume control slider to appear on the screen and the plain UP, DOWN and also LEFT and RIGHT buttons to work directly as long as the slider is on the screen.

E.g. I press-and-hold UP to raise the volume, but then I want to lower it, or to raise it a bit more. If i just press DOWN or UP, it won't work, as these are bind to other actions. I would expect the directional keys to work directly, as long as the slider is on screen, regardless of what app I'm in and what the buttons are linked to otherwise. Maybe also link the PLAY button to Mute and Unmute.

A similar control "mode" may be used for the brightness and other sliders.

Thanks.