On my 4K screens the control buttons of the preview of audio and video files is tiny. I know this is sth Windows provides; but do you know any way to scale that (other than scaling everything else with it)?
We have no control over the scaling (or lack of) in Microsoft's viewers.
Our own Movie viewer should scale its controls properly, but can be a bit of a pain to make work with current versions of Windows (Microsoft seem to have stopped including the needed codecs):
We will be re-writing our Movie player in the future so it works more easily out of the box.
That helped and looks much better 
The lack of codecs was easily fixed by installing a codec-pack. Then I added the missing extensions to the movie-plugin. There is a limitation of how many characters can be entered, but I removed a few of the default ones I know I never need.