Does the mouse have a wheel? And if so, is the wheel free-spinning or latched? If it’s free-spinning, it might be in a position where it can give impulses randomly.
Or if it’s the keyboard, make sure that the cursor down key(s) are not pressed inadvertently by anything.
There are but they are not being triggered, it’s the Plex Media Server itself that is getting muted, the PC itself doesn’t have its sound setting altered in any way.
And yet it’s the Plex Player on our Android Smart TV that mutes every time after half a second, if we then choose the Audio channel again in the video, it will work again for half a minute and then go silent again after about half a minute, etc…
By the way: it’s not all video’s, it’s some of them. And one day a certain video works; another day it’s muted again every 30 seconds or so and we have to keep choosing the Audio via settings again and again.