Ah, that was my first guess yesterday. But I was looking for a missing file in ./Umineko/sound/voice/13 that would start with "1" (as in: chapter 1). And I did find a few files that skipped the n+1 convention, so I tried putting an .ogg file with 1 second of silence in those "gaps", but it didn't do the trick.
What I didn't expect is that the tea parties have a prefix of their own. And sure enough:
ZArchiver must have skipped 90400073.ogg when I was unpacking the archive.
I put in a blank 90400073.ogg file and everything's A-okay now:
All I can say is: thanks for a super quick response. Back to reading and being amazed at the sheer amount of work the team put into this project. Thanks!
Had to make a new account since there's an issue with password recovery, I think. Don't be mad at me, please.