Is the drive mounted? Windows 3.1 in DosBox can work with whatever drives DosBox has.
If so...as I recall from my own install (may be mistaken) Windows didn't install the cd extensions by default, you need to install them. How to do this is beyond the scope of this post.
A full install will, also, break movies for whatever reason (in this particular DosBox scenario), so do a minimal install, that fix was posted soewhere in this forum, I believe.

I don't really know a lot about DOS and drive mounting, so I'm not sure what you mean, sorry 😒 but I don't think I mounted this drive...I run Windows 3.1.1 by opening it as a DOS program.
The folder in which the Windows 3.1.1 file (windows.bat) is located is the same as this Windows' C: drive

There's your problem.
You probably want to read this first.
60 seconds guide to getting your game to run in DOSBox
Read the DosBox (readme) manual about mounting and do proper mountings. At least mount some folder as C and your CD as D.

I finally got it. I was able to mount the drive, so Windows 3.1.1 was able to recognize the D: drive.
I still had the same problem though, I'm pretty sure it's something with the game itself, not DOSBox
or Windows 3.1.1. I'll keep trying to figure a way to make it work, thanks a lot though! I was able
to understand a lot more about drive mounting from that tutorial 😀

If you can start the game and Windows recognizes the drive...
Two things:
-Reinstall the game from within the Win 3.1 running in DosBox and always use the same mountings.
-As I said full installs are for whatever reason bad for Dark Seed 2's videos...do a small install.