Thanks for digging into this.
If I understand correctly, I can fix this issue by setting that flag to true during the creation of the window that causes the video playback to start. I did a quick test on my own, but I'm not able to reproduce the sound carrying through issue on my end at all. In my simple test I have one splash video and nothing else. Hitting a key to skip it, or pressing a mouse button seems to skip it normally with no sound continuing through to the title screen. If I do it with two videos, and skip them both, I get the same - no sound playing through the title screen.
I added a line to set that flag as true in the window that shows the videos immediately when it is created, and that doesn't seem to stop anything from working on the splash screens, and doesn't cause an issue with audio playback, but I never saw the original issue so I don't know that it fixes it either.
Window_DummyWindow.prototype.initialize = function(rect) {
Window_Base.prototype.initialize.call(this, rect);
this._frameCount = Ramza.PTS.params.waitFrames - 12
this._waiting = false
Graphics._videoUnlocked = true
};
Basically I just added the last line to that function in the MV plugin. Try that and see if it changes anything? I suspect it might also cause an issue with video playback elsewhere in game, as the flag will start true unless I also reset it later.