Sadly this time it is still an open question.
Whenever there’s a system beep sound, Windows 10 automatically lowers the volume of everything else for a pretty damned long while (maybe 5-10 seconds). This is particularly annoying when trying to adjust the volume, as adjusting the volume will itself generate a beep, causing the volume to be automatically reduced (WTF were those engineers thinking when then did that? :o).
Anyway, apparently this could be caused by Windows trying to be smart and “guessing” that you are using VoIP, and then reducing the volume of other activities so that you can hear your communication better. The solution, as I found there for instance, would be to disable that feature, by going to system sound settings, then in the “Communications” tab and choosing “When Windows detects communications activity, do nothing” (the default is a huge “reduce the volume of other sounds by 80%” :o).
Sadly, for me it didn’t do the trick, even after a restart (which was suggested there).
I’m running out of ideas to fix this (already), so if you have a magical solution do post it in the comments ^^
Off-topic bonus: apparently Windows 10 comes with an annoyance called Xbox DVR, which doesn’t seem to be necessarily linked to owning an Xbox and can fuck up performance in some games. Steam provides a guide to disable it there.