While there's virtually nothing wrong with the inclusion of the NewDark engine for the original two Thief games, for preservation's sake, I've made some patches that get the original (aka OldDark) engines working on modern systems, this also includes the cutscenes.

What's Included
- Original game executables
- Direct3D and DirectDraw dgVoodoo wrappers for better compatibility on current systems.
- DSOAL for EAX emulation
- NewDark Engine as a backup (named NewDark.exe)

Downloads
Thief Gold Patch

Thief 2 Patch

Installation
1. Extract the content of the patch's "Thief Gold" or "Thief 2" folder to the respective game's install directory. Overwrite the files when it asks you to.
2. Right-click on the new THIEF/THIEF2.EXE, go to the Compatibility tab. Check the box that says "Run this program as an administrator". Hit Apply, then OK.
4. Run THIEF.EXE/THIEF2.EXE and enjoy!!

Cutscenes Fix
If cutscenes don't work, run IV5_FLTR.EXE in the "cutscenes fix" folder. If problem still persists, check the README.TXT in that folder.

PLEASE NOTE
When you launch the game the first time after startup, it may crash, but running it again works without issue.

My patch will not help with that issue, that sounds like something else entirely.