The wheel does work when just hovering over many sliders, at least on Mac and in Firefox. However, whether that works in any particular app or a site is a gamble.
In some apps and sites with a specific purpose the wheel is often mapped more generally: e.g. to volume control in video players regardless of the position, unless it's over the time slider.
In some apps and sites with a specific purpose the wheel is often mapped more generally: e.g. to volume control in video players regardless of the position, unless it's over the time slider.