Thanks for posting this fix up - I did some searching (finally resorting to the Windows search function) to find the guisettings.xml file, which is in a totally different location on Windows installs.
Once I found it, I found the volume setting entry at -3160 and modified it to -200. Seems to have worked. We'll see if it lasts.
I might look at 'normalizing' the volume levels in my various video files, as there is still a lot of variance, but I no longer have to max out the volume on my receiver to hear shows, and then get blasted when playing content through VLC or other players.
I'm running a Windows machine with optical audio out to a receiver, for what it's worth. But none of that seems to be the issue - it's this oddball volume setting parameter.