1. The floppy release. This version has no voice acting and seems to be quite broken. Even in ScummVM you get no sound effects. There seems to be no patch available.
2. The initial version 1 CD-ROM release. There is a v1.04 patch available to address sound issues.
3. The “director’s cut” version 2 CD-ROM release.
You can tell which version you have by looking at the game directory. If you got ACT1.SCN, you’re on the best version, the “director’s cut”. All other versions got files ending in .GRA instead of .SCN
The directors cut seems to be the best, most bugfree way to play. Excerpt from the linked FAQ:
> This is Discworld as it should have been originally.. it fixes virtually all the bugs, graphical and sound glitches in the original version. It also contains a number of new graphical bits, and cut-scenes, such as added animation, and also a host of new sounds.